[CMake] Fix Windows build problem for CMake versions < 3.3

Building libomp using CMake versions < 3.3 caused a link time error.  These
errors occurred because when assembling z_Windows_NT-586_asm.asm, the
definitions: OMPT_SUPPORT, _M_AMD64|_M_IA32 weren't defined on the command line.
To fix the problem, the COMPILE_FLAGS property for the assembly file is appended
to instead of the COMPILE_DEFINITIONS property being set.  For whatever reason, the
COMPILE_DEFINITIONS property doesn't pick up the definitions for assembly files
for the older CMake versions.

llvm-svn: 263651