cc-wrapper: multi-target compilers aren't supported

[mazunki]

Before #2315, we get the following warnings flooding our logs:

Warning: supplying the --target x86_64-pc-linux-elf != x86_64-unknown-linux-musl argument to a nix-wrapped compiler may not work correctly - cc-wrapper is currently not designed with multi-target compilers in mind. You may want to use an un-wrapped compiler instead.

which originate from https://github.com/NixOS/nixpkgs/blob/master/pkgs/build-support/cc-wrapper/add-clang-cc-cflags-before.sh#L29.

The reason this warning appears, I speculate, is that we're building our compiler for one target, and building IncludeOS and our unikernels for another. This is necessary unless we want to build a single compiler for each target we want to work with, which is probably unnecessary.

See #2315 for a further discussion.

[mazunki]

We should check how this behaves on aarch64/arm too.

The main questions to investigate and answer here is what the meaning of -elf and -musl actually is, and how the compiler treats them. I suspect the pc vs unknown piece doesn't matter that much, but the last component could potentially make a difference.