libc stdlib: implement C99 _Exit and C11 quick_exit API

Summary: needed for some newer changes shortly :)

Test Plan: verified

Reviewers: randall.rushing, jriordan001, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D237

Change-Id: I2b34a9cfc2a71721e4bda47b9b06553a096a813c
Signed-off-by: DHOMD <DHO@dho.im>