[OpenMP] Link against libm on OpenBSD (#70614)

Needed for some math functions in libomp.

Author: brad0

openmp/runtime/cmake/LibompHandleFlags.cmake

@@ -140,7 +140,7 @@ function(libomp_get_libflags libflags)
   if(LIBOMP_HAVE_SHM_OPEN_WITH_LRT)
     libomp_append(libflags_local -lrt)
   endif()
-  if(${CMAKE_SYSTEM_NAME} MATCHES "DragonFly|FreeBSD")
+  if(${CMAKE_SYSTEM_NAME} MATCHES "DragonFly|FreeBSD|OpenBSD")
     libomp_append(libflags_local "-Wl,--no-as-needed" LIBOMP_HAVE_AS_NEEDED_FLAG)
     libomp_append(libflags_local "-lm")
     libomp_append(libflags_local "-Wl,--as-needed" LIBOMP_HAVE_AS_NEEDED_FLAG)