Fix llvm-ar error caused by using lto and sanitizer together

LLVM-AR does not allow passing --plugin options more than once. The
--plugin ARFLAGS that lto want to add, may already exist if sanitizer is
also turned on.

Fixed this by adding a new bool Flags.ArGoldPlugin. Set this variable to
true whenever LLVM gold plugin is needed for ArFlags. In function
TransformObjToStaticLib(), add this option to arFlags using global value
${config.LLVMGoldPlugin} if the bool value is true.

Bug: http://b/73160350
Test: build the image with make and succeeded.

Change-Id: I62785829b0a4b663225926e4aed98defc1b6da2c
(cherry picked from commit 4917049f6efe2d08e67eb26ec77e21dcce5172c5)
diff --git a/cc/util.go b/cc/util.go
index 5131b09..92a32bc 100644
--- a/cc/util.go
+++ b/cc/util.go
@@ -84,6 +84,7 @@
 		systemIncludeFlags: strings.Join(in.SystemIncludeFlags, " "),
 
 		groupStaticLibs: in.GroupStaticLibs,
+		arGoldPlugin:    in.ArGoldPlugin,
 	}
 }