libc: Fix longjmp/_longjmp(buf, 0) for MIPS

Like AArch64 and RISC-V in the past, MIPS fails to handle this special
case, and will cause the corresponding setjmp/_setjmp to return 0 rather
than 1. Fix this so the newly-added regression tests pass.

This is a direct commit to stable/13 as mips no longer exists in main.

Reviewed by: arichardson, jhb
Differential Revision: https://reviews.freebsd.org/D29363