`extra_targets`, user configurable toolchains, and wasm support

[rrbutani]

(this PR builds on #75; that PR should be reviewed/merged before this one)

This PR is not ready for merging yet but I thought I'd open the PR anyways to get some feedback and see if this is a use case the maintainers are interested in.

---

This PR adds support for extra_toolchains, an attribute on configure that lets users register additional toolchains for different target triples:

load("@com_grail_bazel_toolchain//toolchain:rules.bzl", "llvm_toolchain")
llvm_toolchain(
    name = "llvm_toolchain",
    llvm_version = "8.0.0",
    extra_targets = [
      "wasm32-unknown-wasi",
    ],

    # Extra targets can have their sysroots overriden too:
    sysroot = {
        "linux": "@some_example_sysroot_repo//:linux_sysroot",
        "darwin": "@some_example_sysroot_repo//:macos_sysroot",

        "linux_wasm32-unknown-wasi": "@some_example_sysroot_repo//:wasi_sysroot",
        "darwin_wasm32-unknown-wasi": "@some_example_sysroot_repo//:wasi_sysroot",
    },
)

The toolchains registered with extra_targets have the appropriate constraint values so that Bazel will use them as necessary through toolchain resolution.

---

In extending the internals of this repo to support extra_targets, cc_toolchain_config and other internal macros were made a fair bit more general. This PR also tweaks these macros to be usable externally, allowing users to make small modifications to the configured toolchains without needing to modify this repo:

# WORKSPACE
# (parts to set up `@llvm_toolchain` have been elided; see above)

llvm_toolchain(
    name = "llvm_toolchain",
    llvm_version = "8.0.0",

    # NOTE: This is required to set up toolchains outside of `@llvm_toolchain`, unfortunately
    absolute_paths = True,
 )

# This registers the default toolchains.
load("@llvm_toolchain//:toolchains.bzl", "llvm_register_toolchains", "register_toolchain")

llvm_register_toolchains()

# Now let's make our own:
http_archive(
    name = "thumbv7-sysroot",
    urls = ["example.com"],
)
register_toolchain("//tests:custom_toolchain_example")

# BUILD file:
# Example Custom Toolchain:
load("@llvm_toolchain//:cc_toolchain_config.bzl", "cc_toolchain_config")

# Docs for this function and `overrides` are in `cc_toolchain_config.bzl.tpl`.
cc_toolchain_config(
    name = "custom_toolchain_example_config",
    host_platform = "linux",
    custom_target_triple = "thumbv7em-unknown-none-gnueabihf",
    overrides = {
        "target_system_name": "thumbv7em-unknown-none-gnueabihf",
        "target_cpu": "thumbv7em",
        "target_libc": "unknown",
        "abi_libc_version": "unknown",

        # If you omit this, be sure to depend on
        # `@llvm_toolchain:host_sysroot_components`.
        # "sysroot_path": "external/thumbv7-sysroot/sysroot",

        "extra_compile_flags": [
            "-mthumb",
            "-mcpu=cortex-m4",
            "-mfpu=fpv4-sp-d16",
            "-mfloat-abi=hard",
        ],
        "omit_hosted_linker_flags": True,
        "omit_cxx_stdlib_flag": False,
        "use_llvm_ar_instead_of_libtool_on_macos": True,
    }
)

load("@com_grail_bazel_toolchain//toolchain:rules.bzl", "conditional_cc_toolchain")
conditional_cc_toolchain(
    name = "custom_toolchain",
    toolchain_config = ":custom_toolchain_example_config",
    host_is_darwin = False,

    sysroot_label = "@llvm_toolchain//:host_sysroot_components", # use this if not overriding
    # sysroot_label = "@thumbv7-sysroot//:sysroot", # override

    absolute_paths = True, # this is required for toolchains set up outside of `@llvm_toolchain`, unfortunately
    llvm_repo_label_prefix = "@llvm_toolchain//",
)

# Constraints come from here: https://github.com/bazelbuild/platforms
toolchain(
    name = "custom_toolchain_example",
    exec_compatible_with = [
        "@platforms//cpu:x86_64",
        "@platforms//os:linux",
    ],
    target_compatible_with = [
        "@platforms//cpu:armv7", # `v7e-mf` has not yet made it to stable Bazel?
        # "@platforms//os:none",
    ],
    toolchain = ":custom_toolchain",
    toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
)

The interface is currently extremely rough and is just intended to be a way to prototype support for other targets without needing to modify this repo. If there's interest, there's lots that can be done to turn this into a user-friendly API.

---

Currently, only the wasm32-unknown-wasi target triple is tested and actually working. However this PR lays the foundation for support for other targets; it should just be a matter of adding sysroot and compiler_rt locations for other targets and fine-tuning the LLVM target triple to Bazel constraint value mappings.

---

As an aside, even in adding support for just a single additional (not-hosted) target the ways in which using unix_cc_toolchain_config.bzl (#75) limit us becomes apparent (we would like to have the toolchain for wasm targets not support the dynamic linking actions since they just error; instead we're forced to use --features=-supports_dynamic_linker on the command line or transitions/selects or other such workarounds).

So far this has been manageable but this definitely casts some doubt on whether #75 is the way to go.

Another thing to consider is whether upstream Bazel would be willing to make unix_cc_toolchain_config.bzl slightly more general/whether or not our use case aligns with its intended purpose.