[UBSan] Fix typo in CMake conditional that checked if the architecture

of a darwin platform was in the list of `UBSAN_SUPPORTED_ARCH`.

This is a follow up to r341306.

The typo meant that if an architecture was a prefix to another
architecture in the list (e.g. `armv7` is a prefix of `armv7k`) then
this would trigger a match which is not the intended behaviour.

rdar://problem/41126835

llvm-svn: 342553