A better x86_64 register mapping, with more argument registers.

A better x86_64 register mapping, with more argument registers.

Now that we can handle using C argument registers as global registers,
extend the x86_64 register mapping.  We now have 5 integer argument
registers, 4 float, and 2 double (all caller-saves).  This results in a
reasonable speedup on x86_64.