aarch64: Run SUBTARGET_INIT_BUILTINS if it exists

Some subtargets don't provide the canonical function names as
the symbol name in C libraries, and libcalls will only work if
the builtins are patched to emit the correct library name.

For example, on NetBSD, cabsl has the symbol name __c99_cabsl,
and the patching is done via netbsd_patch_builtin.

With this change, libgfortran.so is correctly built with a
reference to __c99_cabsl, instead of "cabsl" which is not defined.

gcc/ChangeLog:
	* config/aarch64/aarch64.c (aarch64_init_builtins):
	Call SUBTARGET_INIT_BUILTINS.

gcc/config/aarch64/aarch64.c

@@ -13492,6 +13492,9 @@ aarch64_init_builtins ()
 {
   aarch64_general_init_builtins ();
   aarch64_sve::init_builtins ();
+#ifdef SUBTARGET_INIT_BUILTINS
+  SUBTARGET_INIT_BUILTINS;
+#endif
 }
 
 /* Implement TARGET_FOLD_BUILTIN.  */