Fix the CMake rules for LLVMCFIVerify library not to pull duplicate
LLVM .a libraries when linking to the dylib.  This prevents problems
due to duplicate symbols and apparently fixes mingw32.

This is an alternative approach to D44650 that just forces .a libraries
instead.  However, there doesn't seem to be any reason to do that.

Differential Revision: https://reviews.llvm.org/D81921