[CMake] Fix libomp_check_linker_flag function

libomp_check_linker_flag rewrites src_to_link.c and CMakeLists.txt in build
directory for test project, but cmake does not rebuild the project. The root
cause is that on some filesystems (ext3, reiserfs) timestamp resoultion is 1
second. So cmake does not rebuild test project if check takes less than 1 second.
This patch puts each test in its own directory to avoid the timestamp problem.

Patch by Chris Bergstrom

http://lists.cs.uiuc.edu/pipermail/openmp-dev/2015-July/000817.html

llvm-svn: 243017