riscv64gc-unknown-linux-musl support

[walksanatora]

No description provided.

[walksanatora]

it is appearing to work
but the -Z build-std is not working

[walksanatora]

it fails with

   Compiling nothing v0.0.1 (/home/walksanator/git/rust/oc2/project)
error[E0463]: can't find crate for `panic_abort`

For more information about this error, try `rustc --explain E0463`.
error: could not compile `nothing` due to previous error

[walksanatora]

after specifying -Zbuild-std=std,panic_abort it fails with a new message

  = note: warning: unsupported linker arg: -znoexecstack
          warning: unsupported linker arg: -zrelro
          warning: unsupported linker arg: -znow
          error: static library 'unwind' not found. search paths: 
           /home/walksanator/git/rust/oc2/project/target/riscv64gc-unknown-linux-musl/debug/deps/libunwind.a
           /home/walksanator/git/rust/oc2/project/target/debug/deps/libunwind.a
           /home/walksanator/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/riscv64gc-unknown-linux-musl/lib/libunwind.a
           /home/walksanator/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/riscv64gc-unknown-linux-musl/lib/libunwind.a
           /home/walksanator/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/riscv64gc-unknown-linux-musl/lib/self-contained/libunwind.a
           suggestion: use full paths to static libraries on the command line rather than using -l and -L arguments
           ```

[messense]

Looks like the same kind issue as rust-lang/wg-cargo-std-aware#81