fix sh jmp_buf size to match ABI

fix sh jmp_buf size to match ABI

while the sh port is still experimental and subject to ABI
instability, this is not actually an application/libc boundary ABI
change. it only affects third-party APIs where jmp_buf is used in a
shared structure at the ABI boundary, because nothing anywhere near
the end of the jmp_buf object (which includes the oversized sigset_t)
is accessed by libc.

both glibc and uclibc have 15-slot jmp_buf for sh. presumably the
smaller version was used in musl because the slots for fpu status
register and thread pointer register (gbr) were incorrect and must not
be restored by longjmp, but the size should have been preserved, as
it's generally treated as a libc-agnostic ABI property for the arch,
and having extra slots free in case we ever need them for something is
useful anyway.