[OpenMP][CMake] Ensure linking against libm for Linux

Do the same as is done for NetBSD. Some compiler-rt/lib/builtins files call
libm functions (e.g. fmaxl, fabs). Linking libomp with --rtlib=compiler-rt
references these functions.
Downstream report: https://bugs.gentoo.org/816831

Fixes: https://github.com/llvm/llvm-project/issues/51457