sim-main.h: Re-arange r5900 registers so that they have their own

sim-main.h: Re-arange r5900 registers so that they have their own
little struct.
interp.c: Update.  Also add floating point Max/Min functions.
mips.igen: Remove r5900 tag from any floating point instructions.
r5900.igen: Rewrite.  Implement *all* floating point insns (except ld/st).
r5400.igen: Tag mdmx functions as being mdmx specific.