toolchain: packetize libatomic

Based on a patch by Tobias Steinicke <tobias.steinicke@inet.tu-berlin.de>.

Signed-off-by: Steven Barth <steven@midlink.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42514 3c298f89-4303-0410-b956-a3cf2f4a3e73