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