libc: Fix longjmp/_longjmp(buf, 0) for AArch64 and RISC-V

These architectures fail to handle this special case, and will cause the
corresponding setjmp/_setjmp to return 0 rather than 1. Fix this and add
regression tests (also committed upstream).

PR: 268684
Reviewed by: arichardson, jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D29363