Only build ARM-specific runtimes if ARM is enabled

The soft-float variants of (embedded) libclang_rt only make sense for ARM, so
there's no point in trying to build them if the compiler is only capable of
targeting x86.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197033 91177308-0d34-0410-b5e6-96231b3b80d8