Change mips64 default to mips64r6 from mips64r2

Also add back mips flags previously can't be handled by clang

See aosp/build
  13066a8d4c2f8ddb2348e6d3df3948a0f4dbf105
  10326b3d7f1966381812ef55478619f714917c23

Change-Id: Iedc8dadd24f72fbd9c5589eec34677c1827290f3
diff --git a/core/combo/TARGET_linux-mips64.mk b/core/combo/TARGET_linux-mips64.mk
index 938b11c..4511b47 100644
--- a/core/combo/TARGET_linux-mips64.mk
+++ b/core/combo/TARGET_linux-mips64.mk
@@ -31,7 +31,7 @@
 # version.
 #
 ifeq ($(strip $(TARGET_ARCH_VARIANT)),)
-TARGET_ARCH_VARIANT := mips64r2
+TARGET_ARCH_VARIANT := mips64r6
 endif
 
 # Decouple NDK library selection with platform compiler version