Removal of nightly dependency

[vados-cosmonic]

Context

As discussed in https://github.com/bytecodealliance/WASI-Virt/pull/21, it may be possible to find a way to remove the dependency on Rust Nightly.

This issue represents that effort and any experimentation that may be necessary, with at least a few methods already discussed:

Proposal

There were a couple of options presented:

• Removing -Zoom=panic (likely to cause an increase in code size)
• Avoiding using custom flags in Cargo.toml

While not previously discussed, it seems to be using a build.rs script (emitting cargo:rustc-flags=FLAGS), since profile.rustflags are passed to the compiler (it doesn't look like there's any way to emit anything to pass to the linker on a rust lib target.

Context