Add BUILD_CLIENT_ONLY build variable

Provides support for building a JDK for which only the client VM is required and/or available. Essentially avoids copying any "server" components and creates a jvm.cfg file that aliases -server to -client. If BUILD_CLIENT_ONLY is selected on a 64-bit build then a sanity warning is issued. (This is only a warning because you can actually build 64-bit client Hotspot, it just isn't supported and you can't directly build it in a full JDK build without hacking a couple of Makefiles).