t_signal_and_sp: Try to fix mips o32 tests.

Evidently, for o32, PIC_PROLOGUE (which expands to .cpload) must be
the very first instruction of the function, or else it will get the
$gp calculation wrong.

Since it seems that the mips PIC_PROLOGUE does not, after all, mess
with the stack, let's just dispense with the temporary copy of sp in
t0 which I had added in paranoia over what magic might happen inside
PIC_PROLOGUE. (Such paranoia is not entirely unjustified -- for
example, the sh3 PIC_PROLOGUE _does_ push data on the stack, though
this can be bypassed with PIC_PROLOGUE_NOSAVE.)

PR kern/59327: user stack pointer is not aligned properly

t_signal_and_sp: Add mips support.

PR kern/59327: user stack pointer is not aligned properly

PR kern/58149: Cannot return from a signal handler if SP was
misaligned when the signal arrived

Stack pointer misaligment in some cases hypothesized to be a possible
cause of:

PR port-evbmips/59236: Multiple segfaults in erlite3 boot