Use longjmp in fault_handler_openbsd if siglongjmp unavailable (OpenBSD)

Use longjmp in fault_handler_openbsd if siglongjmp unavailable (OpenBSD)

* os_dep.c [OPENBSD] (GC_jmp_buf_openbsd): Use JMP_BUF instead of
sigjmp_buf.
* os_dep.c [OPENBSD] (GC_fault_handler_openbsd): Use LONGJMP() instead
of siglongjmp().
* os_dep.c [OPENBSD] (GC_find_limit_openbsd, GC_skip_hole_openbsd): Use
SETJMP(buf) instead of sigsetjmp(buf, 1).