MIPS: fix failure to save all argument registers in varargs function

[Alexandre Oliva]

Whenever the last named argument in a function is passed in a
floating-point register, GCC would fail to save any remaining
argument-passing GP registers in their home locations, so the
arguments passed would be lost.  This was demonstrated with
gcc.c-torture/execute/va-arg-16.c with -mabi=o64 -mlong32 -mgp64, with
both -meb and -mel, on a Red Hat-internal MIPS port.

I don't understand why saving varargs registers is handled in the
prologue for some ABIs and in SETUP_INCOMING_VARARGS in others, but
this patch fixes the o64 bug without breaking any other ABIs.  Tested
with the internal port, and bootstrapped with GCC mainline on
mips-sgi-irix6.5.  Eric Christopher approved this patch, so I'm
checking it in.

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-- 
Alexandre Oliva   Enjoy Guarana', see http://www.ic.unicamp.br/~oliva/
Red Hat GCC Developer                  aoliva@{cygnus.com, redhat.com}
CS PhD student at IC-Unicamp        oliva@{lsd.ic.unicamp.br, gnu.org}
Free Software Evangelist                Professional serial bug killer