Fix macos target assumption in test

Clang test Driver/macos-apple-silicon-slice-link-libs-darwin-only.cpp
assumes the target is darwin when the host is darwin which is not
necessarily the case, causing the test to fail when it is not. This
commit adds a -triple argument to the clang invocation to ensure the
target is darwin.

Reviewed By: hans

Differential Revision: https://reviews.llvm.org/D94396