Add support for building for x86_64 platforms.

This allows beebs to be configured and built for x86_64 platforms.
Configure as `./configure --with-chip=generic` after which `make`
should work.

ChangeLog

	* config/x86_64/chips/generic/chip.cfg: New file.
	* config/x86_64/chips/generic/chipsupport.h: New file.