[builtins][CMake] Replace custom target for lse_builtin symlinks

Using custom target with BYPRODUCTS results in symlinks being recreated
on every build (the build is always dirty). We could instead use custom
commands to generate the symlinks and by adding the output to the list
of sources, build system will ensure those are created as needed.

Author: petrhosek

compiler-rt/lib/builtins/CMakeLists.txt

@@ -567,9 +567,13 @@ foreach(pat cas swp ldadd ldclr ldeor ldset)
   foreach(size 1 2 4 8 16)
     foreach(model 1 2 3 4 5)
       if(pat STREQUAL "cas" OR NOT size STREQUAL "16")
+        set(source_asm "${CMAKE_CURRENT_SOURCE_DIR}/aarch64/lse.S")
         set(helper_asm "${OA_HELPERS_DIR}/outline_atomic_${pat}${size}_${model}.S")
-        list(APPEND lse_builtins "${helper_asm}")
-        list(APPEND arm64_lse_commands COMMAND ${CMAKE_COMMAND} -E ${COMPILER_RT_LINK_OR_COPY} "${CMAKE_CURRENT_SOURCE_DIR}/aarch64/lse.S" "${helper_asm}")
+        add_custom_command(
+          OUTPUT "${helper_asm}"
+          COMMAND ${CMAKE_COMMAND} -E ${COMPILER_RT_LINK_OR_COPY} "${source_asm}" "${helper_asm}"
+          DEPENDS "${source_asm}"
+        )
         set_source_files_properties("${helper_asm}"
           PROPERTIES
           COMPILE_DEFINITIONS "L_${pat};SIZE=${size};MODEL=${model}"
@@ -827,16 +831,6 @@ else ()
         list(APPEND BUILTIN_CFLAGS_${arch} -fforce-enable-int128)
       endif()
 
-      if(arch STREQUAL "aarch64")
-        add_custom_target(
-          lse_builtin_symlinks
-          BYPRODUCTS ${lse_builtins}
-          ${arm64_lse_commands}
-        )
-
-        set(deps_aarch64 lse_builtin_symlinks)
-      endif()
-
       add_compiler_rt_runtime(clang_rt.builtins
                               STATIC
                               ARCHS ${arch}