Cmake variables are global, which is why we would get crud like /machine:X86 in the list of compiler options for MSVC. Clear out the variable before attempting to enumerate arguments and set them.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@220522 91177308-0d34-0410-b5e6-96231b3b80d8