Fix simple cmake error when COMPILER_RT_SUPPORTED_ARCH is empty.
Fix simple cmake error when COMPILER_RT_SUPPORTED_ARCH is empty.

Differential Revision: https://reviews.llvm.org/D27719

git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@289749 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt