Cross-compiling with clang and (c)dylibs in the dependency graph is broken

[jschwe]

Bug Scenario:

• Cross-compiling
• Linker driver is clang (i.e. requires --target=triple option to link for the correct target)
• OS is not MacOS (rustc adds --target automatically on Darwin).
• The dependency graph contains a target that requires the linker to be invoked for the cross-compilation target. This should only be dynamic libraries. Note: This does not include the final artifact - We already pass the the --target flag via cargo rustc to the final linker invocation.

Untested, but probably this requirement also applies:

• compiler option to use lld (-fuse-ld=lld) is NOT set.

If such a dependency exists in the graph, then linking it will fail, since clang will call the host linker instead of the target linker.

Constraints for solutions:

• Adding the argument to the global RUSTFLAGS has the side-effect of overriding any rustflags set via .cargo/config.toml, which may be unexpected, especially since it only happens in the cross-compiling with clang scenario.
• Setting the linker to clang --target=xxx does not work, since this is treated as the executable name.
• Writing a small wrapper shell script would be possible on Linux, but on Windows it looks like there is no guaranteed way to pass through arguments without changing them with a small script. We could instead compile a small Rust program which calls the linker-driver without going over the shell again.
• It is unclear if this will be fixed upstream in Rust, since currently gcc and clang are treated entirely the same (except on mac).