Add DragonFlyBSD support to OpenMP

Summary:
Additions mostly follow FreeBSD and NetBSD and are not intrusive.
There is similar patch for OpenBSD: https://reviews.llvm.org/D34280

The -lm was being omitted due to -Wl,--as-needed in cmake rule, similar patch is in freebsd-ports/devel/llvm-devel port.

Simple OpenMP programs compile and work as expected:
$ clang-devel ~/omp_hello.c -fopenmp -I/usr/local/llvm-devel/include
$ LD_LIBRARY_PATH=/usr/local/llvm-devel/lib OMP_NUM_THREADS=100 ./a.out

The assertion in LLVMgold.so when -fopenmp was used together with -flto in 20170524 snapshot is no longer triggered on current svn-trunk and works fine as in llvm-4.0 with our local patches.

Reviewers: #openmp, krytarowski

Reviewed By: krytarowski

Subscribers: dexonsmith, jfb, krytarowski, guansong, gregrodgers, emaste, mgorny, mehdi_amini

Differential Revision: https://reviews.llvm.org/D35129

llvm-svn: 348725