[OPENMP] -fopenmp enables OpenMP support (fix for http://llvm.org/PR23492)

-fopenmp turns on OpenMP support and links libiomp5 as OpenMP library. Also there is -fopenmp={libiomp5|libgomp} option that allows to override effect of -fopenmp and link libgomp library (if -fopenmp=libgomp is specified).
Differential Revision: http://reviews.llvm.org/D9736

llvm-svn: 237769