ms-inline-asm: Correctly mark MS inline ASM labels as used

Summary: This fixes PR21155.

Test Plan: The patch includes a test.

Reviewers: rnk

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5619
AArch64: add support for ARM64 iOS versions of compiler-rt.

Just a dummy directory and a few sane choices in the Darwin SDK.

rdar://problem/18575597