Add stdatomic.h as symlink to bionic version.

This is basically a cherrypick of commit
fe610b19e10b4db855c69f42480b58f0ff7f2d9a, but the context is
different, and prior versions of the file didn't exist here.
Enables fixes for b/16880454 and

Bug:16513433
Change-Id: I2ec4ed14357967431c6bbb3205ccfc910e549f8e
diff --git a/sysroot/usr/include/stdatomic.h b/sysroot/usr/include/stdatomic.h
new file mode 120000
index 0000000..b79fbfe
--- /dev/null
+++ b/sysroot/usr/include/stdatomic.h
@@ -0,0 +1 @@
+../../../../../../../../bionic/libc/include/stdatomic.h
\ No newline at end of file