art: use -O1 to compile tests with clang for arm64

arm64 clang fails to compile art/runtime/arch/stub_test.cc with:
art/runtime/arch/stub_test.cc:178:9: error: inline assembly requires
more registers than available
        "sub sp, sp, #64\n\t"
        ^

Pass -O1 to all art tests compiled with clang for arm64.

Bug: 26275713
Change-Id: Ib1a1c309aef73a2d33d37d720e5e11b8fe7a248e
diff --git a/build/Android.gtest.mk b/build/Android.gtest.mk
index 291a69d..1c23929 100644
--- a/build/Android.gtest.mk
+++ b/build/Android.gtest.mk
@@ -564,6 +564,9 @@
     LOCAL_MODULE_PATH_64 := $$(ART_TARGET_NATIVETEST_OUT)/$$(ART_TARGET_ARCH_64)
     LOCAL_MULTILIB := both
     LOCAL_CLANG_CFLAGS += -Wno-used-but-marked-unused -Wno-deprecated -Wno-missing-noreturn  # gtest issue
+    # clang fails to compile art/runtime/arch/stub_test.cc for arm64 without -O1
+    # b/26275713
+    LOCAL_CLANG_CFLAGS_arm64 += -O1
     include $$(BUILD_EXECUTABLE)
     library_path :=
     2nd_library_path :=