libc: support __errno_location

Summary:
Android's errno behavior is the same as glibc's __errno_location.
Support __errno_location for compatibility.

Test Plan: verified fully

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D236

Change-Id: I2f48168d31a870836271319460e3a941c6498022
Signed-off-by: DHOMD <DHO@dho.im>