[MIPS64] Modify the default MIPS64 build to MIPS64R6.

Change-Id: I797e4c2e5b69c6991545c26886b5b25e6f8c2a67
diff --git a/core/combo/TARGET_linux-mips64.mk b/core/combo/TARGET_linux-mips64.mk
index b5555b2..a2e64c5 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