[CMake] Add -fno-experimental-isel for testing

GlobalISel doesn't yet implement blockaddress and falls back to
SelectionDAG. This results in additional branch instruction to
the next basic block which breaks the OMPT tests.
Disable GlobalISel for now when compiling the tests because fixing
them is not easily possible. See http://llvm.org/PR36313 for full
discussion history.

Differential Revision: https://reviews.llvm.org/D43195

llvm-svn: 325218