Allow setting platform constraints with --exec_constraints

[coeuvre]

By default, clang is detected for cpp configs. While --cpp_env_json can be used to override the default environment variables during toolchain generation, there isn't a way to override platform constraints.

This PR adds flag --exec_constraints which accept a comma separated list of constraint values e.g. --exec_constraints=@bazel_tools//platforms:linux,@bazel_tools//platforms:x86_64,@bazel_tools//tools/cpp:gcc to override the default values.

[crackcomm]

I would be delighted to have this feature.

[coeuvre]

I have rebased the PR. Please review and let me know whether it's feasible to merge!