Enable the asm_fast* (and reg*) grades on x86_64 OS X machines.

configure.ac:
	Do not set the LLDS base grade to 'none' if we are on 64-bit OS X and
	we are using GCC 4.3 onwards.  (Older versions of GCC are either
	broken or do not work on this platform anyway.)

README.MacOS:
	Mention that the asm_fast* and reg* grades do work on x86_64
	systems.
NEWS:
	Announce that the asm_fast* and reg* grades are now supported
	on x86_64 OS X machines.