fix size of mips jmp_buf

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.