build: always add support for assembly targets on Darwin

When attempting to build compiler-rt on a developer transition kit, the
build would fail due to `.S` files not being handled properly by the
Ninja generator.  Rather than conditionalising on Xcode, conditionalise
to Darwin.  Because we know that the system compiler is clang based, it
will always properly handle the pre-processing based on the extension.

Differential Revision: https://reviews.llvm.org/D84333