Don't build muloti4 on Apple-ARM64.

It looks like it's already included in:
lib/clang/3.8.0/lib/darwin/libclang_rt.ios.a(muloti4.c.o)

Author: nadavrot

stdlib/public/stubs/Stubs.cpp

@@ -258,8 +258,7 @@ extern "C" long double _swift_fmodl(long double lhs, long double rhs) {
 // This implementation is copied here to avoid a new dependency
 // on compiler-rt on Linux.
 // FIXME: rdar://14883575 Libcompiler_rt omits muloti4
-#if (defined(__APPLE__) && defined(__arm64__)) || \
-    (defined(__linux__) && defined(__x86_64__)) || \
+#if (defined(__linux__) && defined(__x86_64__)) || \
     (defined(__linux__) && defined(__aarch64__)) || \
     (defined(__linux__) && defined(__powerpc64__))