Fix GNU ObjC ABI for a message returning a struct.

This allows the ObjFW runtime to correctly implement message forwarding
for messages which return a struct.

Patch by Jonathan Schleifer.
Fix gcc search for cross-compiler on Ubuntu 13.04.

Just more craziness to find crtbegin.o etc.

Patch by Stephen Kelly.
Tighten type-checking for vector attributes.

Based on patch by Yunzhong Gao.