Vary non-image Xmx for dex2oat on MIPS.

The image Xmx is already fixed at 64m. The issue is in the non boot image case.

Change-Id: Idd508187ce8ac3d3c8dbe357c74bc33a5c8185dd
diff --git a/core/dex_preopt_libart.mk b/core/dex_preopt_libart.mk
index 7b75426..2d9c518 100644
--- a/core/dex_preopt_libart.mk
+++ b/core/dex_preopt_libart.mk
@@ -39,7 +39,7 @@
 # via a property as we want the larger Xmx value if we're running on a
 # MIPS device.
 LIBART_IMG_TARGET_BASE_ADDRESS := 0x30000000
-DEX2OAT_IMAGE_XMX := 128m
+DEX2OAT_XMX := 128m
 endif
 
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