make configure accept alternate gcc tuples for x32

the previous pattern required "x32" to be used as the second field of
the gcc tuple, which is usually reserved for vendor use and not
appropriate as an ABI specifier. with this change, putting "x32" at
the end of the tuple, the way ABI specifiers are normally done, is
also permitted.
use syscall_arg_t for arguments in public syscall() function

on x32, this change allows programs which use syscall() with pointers
or 64-bit values as arguments to work correctly, i.e. without
truncation or incorrect sign extension. on all other supported archs,
syscall_arg_t is defined as long, so this change is a no-op.
remove useless and incorrect uc_regspace member from mips ucontext_t

this seems to have been copied erroneously from the arm version of the
file. it's fairly harmless but it's a mistake and better to fix now
than later.
fix size of mips jmp_buf

the excess space was unused and unintentional. this change does not
affect the ABI between applications and libc. while it does
theoretically affect linkage between third-party translation units
using jmp_buf as part of a structure, we've already changed jmp_buf at
least once on all archs, and problems were never observed, likely
because such usage would be very unusual. in any case it's best to get
things right now rather than making changes sometime during the 1.0.x
series or later.
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.