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/builder.go b/cc/builder.go
index 0646132..279c1da 100644
--- a/cc/builder.go
+++ b/cc/builder.go
@@ -256,6 +256,7 @@
 	systemIncludeFlags string
 
 	groupStaticLibs bool
+	arGoldPlugin    bool
 
 	stripKeepSymbols       bool
 	stripKeepMiniDebugInfo bool
@@ -512,6 +513,9 @@
 	if !ctx.Darwin() {
 		arFlags += " -format=gnu"
 	}
+	if flags.arGoldPlugin {
+		arFlags += " --plugin ${config.LLVMGoldPlugin}"
+	}
 	if flags.arFlags != "" {
 		arFlags += " " + flags.arFlags
 	}