[sanitizer] Cache results of darwin_test_archs

For OS X builds of compiler-rt, we run `darwin_test_archs` to determine which architectures can the toolchain target. This detection takes quite a long time, and the result is always the same (as long as you don't upgrade your OS, system headers or toolchain). Let's cache the result.

Differential Revision: http://reviews.llvm.org/D15179

llvm-svn: 254618