[AArch64] don't build out of line atomics on Darwin or Windows

[t.p.northover]

Summary

The lse.S file uses syntax that only works on ELF targets (e.g. :lo12), so it prevents building compiler-rt for iOS and ARM-based macOS.

[mstorsjo]

The lse.S file uses syntax that only works on ELF targets (e.g. :lo12:)

That syntax actually is supported by the llvm assembler for windows targets, and these out of line functions at least build successfully for that target (haven't tested actually running them though), so I think only darwin targets are the one that need to be excluded here right now.

[ilinpv]

lse.S syntax could be fixed: https://reviews.llvm.org/D92530
The question is, should outline atomics be in compiler-rt for iOS and ARM-based macOS?

[ilinpv]

lse.S syntax fixed, compiler-rt with outline atomics should be built fine. If there are no any other issues can we abandon this patch?

compiler-rt/lib/builtins/CMakeLists.txt

 if(NOT ANDROID)
   append_list_if(COMPILER_RT_HAS_VISIBILITY_HIDDEN_FLAG -DVISIBILITY_HIDDEN CUSTOM_FLAGS)
 endif()
 append_list_if(COMPILER_RT_HAS_ASM_LSE -DHAS_ASM_LSE CUSTOM_FLAGS)
 string(REPLACE " " "\t" CUSTOM_FLAGS "${CUSTOM_FLAGS}")
 set(OA_HELPERS_DIR "${CMAKE_CURRENT_BINARY_DIR}/outline_atomic_helpers.dir")
 file(MAKE_DIRECTORY ${OA_HELPERS_DIR})
 
+if(NOT APPLE AND NOT WIN32)
   foreach(pat cas swp ldadd ldclr ldeor ldset)
     foreach(size 1 2 4 8 16)
       foreach(model 1 2 3 4)
         if(pat STREQUAL "cas" OR NOT size STREQUAL "16")
           set(helper_asm ${OA_HELPERS_DIR}/outline_atomic_${pat}${size}_${model}.S)
           add_custom_command(
             OUTPUT ${helper_asm}
             COMMAND ${CMAKE_C_COMPILER} -E ${CUSTOM_FLAGS} -DL_${pat} -DSIZE=${size} -DMODEL=${model}
                     ${CMAKE_CURRENT_SOURCE_DIR}/aarch64/lse.S -o ${helper_asm}
             DEPENDS aarch64/lse.S assembly.h
           )
           set_source_files_properties(${helper_asm} PROPERTIES GENERATED TRUE)
           set(aarch64_SOURCES
             ${aarch64_SOURCES}
             ${helper_asm}
           )
         endif()
       endforeach(model)
     endforeach(size)
   endforeach(pat)
+endif()
 
 if (MINGW)
   set(aarch64_SOURCES
     ${aarch64_SOURCES}
     aarch64/chkstk.S
   )
 endif()