[compiler-rt] Fix target architecture matching

Summary:
Use stricter comparisons for architecture. This prevents cmake from failing
for sysroots which can only compile armhf and not arm, since
arm MATCHES armhf is true, while arm STREQUAL armhf is false.

Reviewers: beanz, compnerd

Subscribers: aemerson, llvm-commits

Differential Revision: https://reviews.llvm.org/D22473

llvm-svn: 276148