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

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

(cherry picked from commit 9fb118bebced1452a46756a13be0161021b10905)