mk: Pass exact compile to rustc on cross compiles

Instead of passing through CC which may have things like ccache and other
arguments (when using clang) this commit filters out the necessary arguments
from CC to pass the right linker to rustc.

Closes #13562
auto merge of #13631 : alexcrichton/rust/fix-mk-cross, r=brson

Instead of passing through CC which may have things like ccache and other
arguments (when using clang) this commit filters out the necessary arguments
from CC to pass the right linker to rustc.

Closes #13562