feat: add x86_64-unknown-linux-musl target, specifying target to make

[G4Vi]

The x86_64-unknown-linux-musl target does not build dynamic libraries for now:

 warning: dropping unsupported crate type `cdylib` for target `x86_64-unknown-linux-musl`

A target may now be optionally specified to the root Makefile, example:
make RUST_TARGET=x86_64-unknown-linux-musl && sudo make RUST_TARGET=x86_64-unknown-linux-musl install

The new musl targets may be used to create entirely static binaries with libextism. See the new musl-static target in libextism/Makefile

[zshipko]

Thanks!

I think the CLI is almost set up to handle this too, I need to take a look but I think we can just add a libc flag to specify which libc implementation/compiler to use.

[G4Vi]

I think the CLI is almost set up to handle this too, I need to take a look but I think we can just add a libc flag to specify which libc implementation/compiler to use.

Sounds good, maybe lib versions should list which triplets are available for the current system for each version.