Really use GCC for ARM64

Restores change https://android-review.googlesource.com/98911 lost in change
https://android-review.googlesource.com/98456.

Change-Id: Ie11f807c554dbfa812a70b8a183966f388018276
diff --git a/build/Android.common_build.mk b/build/Android.common_build.mk
index 21ba962..5cfdbfd 100644
--- a/build/Android.common_build.mk
+++ b/build/Android.common_build.mk
@@ -121,10 +121,10 @@
   ART_HOST_CLANG := true
 endif
 
-# Clang on the target: only enabled for ARM64. Target builds use GCC by default.
+# Clang on the target. Target builds use GCC by default.
 ART_TARGET_CLANG :=
 ART_TARGET_CLANG_arm :=
-ART_TARGET_CLANG_arm64 := true
+ART_TARGET_CLANG_arm64 :=
 ART_TARGET_CLANG_mips :=
 ART_TARGET_CLANG_x86 :=
 ART_TARGET_CLANG_x86_64 :=