Revert "<sys/cdefs.h>: Fix clang __used__ attribute handling"

Summary:
This commit isn't correct.
This reverts commit 02bdd3f81d4f6e911e90856a26dfbc1eb57fb203.

Test Plan: verified

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D42