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
Include required definitions from stdint.h

Bug:16874785
Change-Id: If7fddb444c2a0669c9f32d1031e93ce4fd9dce12
Include stddef.h in stdatomic.h to define ptrdiff_t

Bug:16874785
Change-Id: I0056a040ead61f32287567449c83025d8fe4df8d
Replace stdatomic.h with the bionic version.

Incorporates several bug fixes and makes it compatible with <atomic>
from libcxx.

Change-Id: I62547dbafb48ec2c563e199cbf121e65ca5e9d53
Change stdatomic.h to symlink

Change-Id: Id2f549748fb851db7dfd032744f32a4015d2f493