[OpenMP] Use fopenmp prefix for all options introduced by the offloading implementation.

Summary: This patch changes the options used by offloading to start with -fopenmp instead of -fomp. This makes the option naming more consistent and materializes a suggestion by Richard Smith in http://reviews.llvm.org/D9888.

Reviewers: hfinkel, carlo.bertolli, arpith-jacob, ABataev

Subscribers: kkwli0, cfe-commits, caomhin

Differential Revision: http://reviews.llvm.org/D21841

llvm-svn: 274283