libc: further centralize syscall symbols

All architectures necessarily implement _exit(2) and vfork(2) so
declare them in sys/Symbol.map.

Reviewed by: imp, kib, emaste
Differential Revision: https://reviews.freebsd.org/D42614

(cherry picked from commit e4a1800f06884dc00931f55d0fa8cd9ce473a83e)