Add custom <stdatomic.h> to prebuilt gcc-4.8 sysroot.

This patch adds a <stdatomic.h> header, which is a modified version
of the FreeBSD header used by bionic/libc/include/stdatomic.h.

Summary of changes:

- Removal of __has_feature() calls.
- Removal of __unused macro (which is actually not needed here).

BUG=15211001

Change-Id: I54aab3751d272dd0025ef9d64e7c2ee2409e889c