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

[telans]

Summary

Do the same is as done for NetBSD. Compiling libomp with Clang uses fmaxl.
Downstream report: https://bugs.gentoo.org/816831

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

[MaskRay]

Compiling libomp with Clang uses fmaxl.

Some compiler-rt/lib/builtins files call libm functions (e.g. fmaxl, fabs). Linking libomp with --rtlib=compiler-rt references these functions.

After D91841 (@rprichard , compiler-rt/lib/builtins still references fmaxl (x86 fp80) and fabs.

[MaskRay]

Close by 2d0c9b64a07c6c430ccfe11ea4c767d788d20461

openmp/runtime/cmake/LibompHandleFlags.cmake

function(libomp_get_libflags libflags)
   endif()
   if(LIBOMP_HAVE_SHM_OPEN_WITH_LRT)
     libomp_append(libflags_local -lrt)
   endif()
   if(${CMAKE_SYSTEM_NAME} MATCHES "DragonFly|FreeBSD")
     libomp_append(libflags_local "-Wl,--no-as-needed" LIBOMP_HAVE_AS_NEEDED_FLAG)
     libomp_append(libflags_local "-lm")
     libomp_append(libflags_local "-Wl,--as-needed" LIBOMP_HAVE_AS_NEEDED_FLAG)
-  elseif(${CMAKE_SYSTEM_NAME} MATCHES "NetBSD")
+  elseif(${CMAKE_SYSTEM_NAME} MATCHES "Linux|NetBSD")
     libomp_append(libflags_local -lm)
   endif()
   set(libflags_local ${libflags_local} ${LIBOMP_LIBFLAGS})
   libomp_setup_flags(libflags_local)
   set(${libflags} ${libflags_local} PARENT_SCOPE)
 endfunction()
 
 # Fortran flags