[xbuild] Honor MONO_GAC_PREFIX in AssemblyResolver.

This commit adds the GAC directories specified in MONO_GAC_PREFIX to the AssemblyResolver's implicit search path, allowing it to automatically resolve binaries that are in secondary GACs.

This patch assumes that the order in which the GAC paths are specified does not matter. From what I can tell, the AssemblyResolver always uses the newest file if duplicates are found, so this assumption should be valid - but I might be missing something.