libc: centralize {_,sig,}{set,long}jmp symbols

These symbols are universally exposed and documented so declare them
centrally. Double- and triple-underscore versions exist on some
platforms, but leave those alone for now.

Reviewed by: imp, kib
Differential Revision: https://reviews.freebsd.org/D42616

(cherry picked from commit 1c656143be734de4b1e1bac27c0c48ef4d974510)