AArch64: don't just hack arm64 support into compiler-rt.

We should check whether the compiler & linker support ARM64 before attempting
to build a version of compiler-rt for it. Otherwise things will go badly.

rdar://problem/18575597

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@219332 91177308-0d34-0410-b5e6-96231b3b80d8