Expose Clang wrapper's dump usage stats option

This allows us to track how much time is spent in Clang.

Test: TOOLCHAIN_RUSAGE_OUTPUT=/tmp/rusage.txt m
Change-Id: Ib2961904f363bc59bd9d928bb055a96740cb9f17
diff --git a/ui/build/dumpvars.go b/ui/build/dumpvars.go
index 11311f9..285f156 100644
--- a/ui/build/dumpvars.go
+++ b/ui/build/dumpvars.go
@@ -205,6 +205,9 @@
 		"CCACHE_SLOPPINESS",
 		"CCACHE_BASEDIR",
 		"CCACHE_CPP2",
+
+		// LLVM compiler wrapper options
+		"TOOLCHAIN_RUSAGE_OUTPUT",
 	}
 
 	allVars := append(append([]string{