fix typo in filename used in sh port
fix mips sigsetjmp asm to match fixed jmp_buf size

this was missed in the previous commit.
move signal.h definition of stack_t to arch-specific bits

it's different at least on mips. mips version will be fixed in a
separate commit to show the change.
fix mips stack_t

like almost everything on mips, this is gratuitously different.
fix signal.h breakage from moving stack_t to arch-specific bits

in the previous changes, I missed the fact that both the prototype of
the sigaltstack function and the definition of ucontext_t depend on
stack_t.
configure: check for __ILP32__ if arch is x86_64

otherwise a multilib compiler used with -mx32 will not be detected
properly.