[gn build] fix up arm64 builtin sources a bit

The fp_mode.c removal is done by filter_builtin_sources in the cmake build.

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

@@ -427,7 +427,11 @@ static_library("builtins") {
   }
 
   if (current_cpu == "arm64") {
-    sources += [ "aarch64/fp_mode.c" ]
+    sources -= [ "fp_mode.c" ]
+    sources += [
+      "cpu_model.c",
+      "aarch64/fp_mode.c",
+    ]
     if (current_os == "mingw") {
       sources += [ "aarch64/chkstk.S" ]
     }