[OpenMP] Remove the workaround of passing "-x assembler-with-cpp" manually

[mstorsjo]

Summary

By building the assembly with language ASM now, this shouldn't be
needed.

[tianshilei1992]

I guess this patch has to be deferred as the previous one was reverted.

[mstorsjo]

The CMake version requirement patch has been landed and stably remained in main now for many weeks, so I believe this should be ready to proceed again.

[tianshilei1992]

Okay. Then let's go for it.

openmp/runtime/cmake/LibompHandleFlags.cmake

function(libomp_get_cxxflags cxxflags)
   set(cxxflags_local ${flags_local} ${LIBOMP_CXXFLAGS})
   libomp_setup_flags(cxxflags_local)
   set(${cxxflags} ${cxxflags_local} PARENT_SCOPE)
 endfunction()
 
 # Assembler flags
 function(libomp_get_asmflags asmflags)
   set(asmflags_local)
-  libomp_append(asmflags_local "-x assembler-with-cpp" LIBOMP_HAVE_X_ASSEMBLER_WITH_CPP_FLAG)
   # Architectural assembler flags
   if(${IA32})
     if(CMAKE_SIZEOF_VOID_P EQUAL 8)
       libomp_append(asmflags_local -m32 LIBOMP_HAVE_M32_FLAG)
     endif()
     libomp_append(asmflags_local /safeseh LIBOMP_HAVE_SAFESEH_MASM_FLAG)
     libomp_append(asmflags_local /coff LIBOMP_HAVE_COFF_MASM_FLAG)
   elseif(${MIC})

openmp/runtime/cmake/config-ix.cmake

if(WIN32)
   foreach(libomp_lang IN ITEMS C CXX)
     foreach(libomp_btype IN ITEMS DEBUG RELWITHDEBINFO RELEASE MINSIZEREL)
       string(REPLACE "/MD" "/MT"
         CMAKE_${libomp_lang}_FLAGS_${libomp_btype}
         "${CMAKE_${libomp_lang}_FLAGS_${libomp_btype}}"
       )
     endforeach()
   endforeach()
-else()
-  # It is difficult to create a dummy assembly file that compiles into an
-  # executable for every architecture and then check the C compiler to
-  # see if -x assembler-with-cpp exists and works, so we assume it does for non-Windows.
-  set(LIBOMP_HAVE_X_ASSEMBLER_WITH_CPP_FLAG TRUE)
 endif()
 if(${LIBOMP_FORTRAN_MODULES})
   libomp_check_fortran_flag(-m32 LIBOMP_HAVE_M32_FORTRAN_FLAG)
 endif()
 
 # Check for Unix shared memory
 check_symbol_exists(shm_open "sys/mman.h" LIBOMP_HAVE_SHM_OPEN_NO_LRT)
 if (NOT LIBOMP_HAVE_SHM_OPEN_NO_LRT)