gn build: pacify "gn format" after 371102

llvm-svn: 371858

llvm/utils/gn/secondary/compiler-rt/lib/builtins/BUILD.gn

@@ -359,6 +359,8 @@ static_library("builtins") {
 
   if (target_cpu == "arm64") {
     sources += [
+      # This comment prevents `gn format` from putting the file on the same line
+      # as `sources +=`, for sync_source_lists_from_cmake.py.
       "aarch64/fp_mode.c",
     ]
     if (target_os == "mingw") {