[OpenMP][cmake] Simplify -m32 handling (#142742)

Linking `libomp.so` on 32-bit SPARC `FAIL`s with
```
ld: fatal: file projects/openmp/runtime/src/CMakeFiles/omp.dir/z_Linux_asm.S.o: wrong ELF class: ELFCLASS64
```
This was a 1-stage build with a 64-bit-default `gcc`. Unlike the C++
sources, which were compiled as 32-bit objects due to the use of
`-DCMAKE_CXX_FLAGS=-m32`, the assembler sources were not.

This patch simplifies passing `-m32`: instead of doing it per
architecture, `-m32` is now always passed when the target uses 32-bit
pointers and supports the option.

Tested on `sparc-sun-solaris2.11`, `sparcv9-sun-solaris2.11`,
`sparc-unknown-linux-gnu`, `sparc64-unknown-linux-gnu`,
`i386-pc-solaris2.11`, `amd64-pc-solaris2.11`, `i686-pc-linux-gnu`, and
`x86_64-pc-linux-gnu`.

Author: rorth

openmp/runtime/cmake/LibompHandleFlags.cmake

@@ -60,10 +60,10 @@ function(libomp_get_cxxflags cxxflags)
     libomp_append(flags_local -Qinline-min-size=1 LIBOMP_HAVE_INLINE_MIN_SIZE_FLAG)
   endif()
   # Architectural C and C++ flags
+  if(CMAKE_SIZEOF_VOID_P EQUAL 4)
+    libomp_append(flags_local -m32 LIBOMP_HAVE_M32_FLAG)
+  endif()
   if(${IA32})
-    if(CMAKE_SIZEOF_VOID_P EQUAL 8)
-      libomp_append(flags_local -m32 LIBOMP_HAVE_M32_FLAG)
-    endif()
     libomp_append(flags_local /arch:SSE2 LIBOMP_HAVE_ARCH_SSE2_FLAG)
     libomp_append(flags_local -msse2 LIBOMP_HAVE_MSSE2_FLAG)
     libomp_append(flags_local -falign-stack=maintain-16-byte LIBOMP_HAVE_FALIGN_STACK_FLAG)
@@ -81,10 +81,10 @@ endfunction()
 function(libomp_get_asmflags asmflags)
   set(asmflags_local)
   # Architectural assembler flags
+  if(CMAKE_SIZEOF_VOID_P EQUAL 4)
+    libomp_append(asmflags_local -m32 LIBOMP_HAVE_M32_FLAG)
+  endif()
   if(${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(${MIC})
@@ -112,10 +112,10 @@ function(libomp_get_ldflags ldflags)
   libomp_append(ldflags_local -static-intel LIBOMP_HAVE_STATIC_INTEL_FLAG)
   libomp_append(ldflags_local /SAFESEH LIBOMP_HAVE_SAFESEH_FLAG)
   # Architectural linker flags
+  if(CMAKE_SIZEOF_VOID_P EQUAL 4)
+    libomp_append(ldflags_local -m32 LIBOMP_HAVE_M32_FLAG)
+  endif()
   if(${IA32})
-    if(CMAKE_SIZEOF_VOID_P EQUAL 8)
-      libomp_append(ldflags_local -m32 LIBOMP_HAVE_M32_FLAG)
-    endif()
     libomp_append(ldflags_local -msse2 LIBOMP_HAVE_MSSE2_FLAG)
   elseif(${MIC})
     libomp_append(ldflags_local -mmic LIBOMP_HAVE_MMIC_FLAG)
@@ -159,7 +159,7 @@ endfunction()
 # Fortran flags
 function(libomp_get_fflags fflags)
   set(fflags_local)
-  if(${IA32})
+  if(CMAKE_SIZEOF_VOID_P EQUAL 4)
     libomp_append(fflags_local -m32 LIBOMP_HAVE_M32_FORTRAN_FLAG)
   endif()
   set(fflags_local ${fflags_local} ${LIBOMP_FFLAGS})