[clang, test, Darwin] Fix tests expecting Darwin target

Clang tests Driver/apple-arm64-arch.c and
Driver/darwin-warning-options.c test Darwin driver functionality but
only require the host system to be Darwin. This leads the tests to fail
when building a cross-compiler on Darwin and to be marked unsupported
when cross-compiling to Darwin from another system. This commit changes
the requirements for those tests to require the target to be Darwin.

Reviewed By: steven_wu

Differential Revision: https://reviews.llvm.org/D85367