Use xz prebuilt

Instead of whichever xz happens to be on the PATH.

Bug: 36130900
Test: prebuilts/build-tools/build-prebuilts.sh
Change-Id: Ic9b66062ca7947f0eca9d839c0864da492d4ef71

cc/builder.go

@@ -109,13 +109,14 @@ var (
 		"objcopyCmd", "prefix")
 
 	_ = pctx.SourcePathVariable("stripPath", "build/soong/scripts/strip.sh")
+	_ = pctx.SourcePathVariable("xzCmd", "prebuilts/build-tools/${config.HostPrebuiltTag}/bin/xz")
 
 	strip = pctx.AndroidStaticRule("strip",
 		blueprint.RuleParams{
 			Depfile:     "${out}.d",
 			Deps:        blueprint.DepsGCC,
-			Command:     "CROSS_COMPILE=$crossCompile $stripPath ${args} -i ${in} -o ${out} -d ${out}.d",
-			CommandDeps: []string{"$stripPath"},
+			Command:     "CROSS_COMPILE=$crossCompile XZ=$xzCmd $stripPath ${args} -i ${in} -o ${out} -d ${out}.d",
+			CommandDeps: []string{"$stripPath", "$xzCmd"},
 		},
 		"args", "crossCompile")