Use xz prebuilt

Instead of whichever xz happens to be on the PATH.

Bug: 36130900
Test: m
Change-Id: Ib37d8db4e935ef8f69f3d43b5065c22d34028cf3
diff --git a/core/definitions.mk b/core/definitions.mk
index d3d9928..b0b92de 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -1853,7 +1853,7 @@
   $(PRIVATE_OBJCOPY) -S --remove-section .gdb_index --remove-section .comment --keep-symbols=$@.keep_symbols $@.mini_debuginfo && \
   $(PRIVATE_OBJCOPY) --rename-section saved_debug_frame=.debug_frame $@.mini_debuginfo && \
   rm -f $@.mini_debuginfo.xz && \
-  xz $@.mini_debuginfo && \
+  $(XZ) $@.mini_debuginfo && \
   $(PRIVATE_OBJCOPY) --add-section .gnu_debugdata=$@.mini_debuginfo.xz $@; \
 else \
   cp -f $< $@; \