cc-tool: pass -latomic in LIBS

Just like -lpthread was passed in LIBS, -latomic should also be passed
in LIBS. In order for this to work, we however need to first fix
cc-tool's Makefile.am so that it does not overwrite LIBS.

This is the first part of fixing the build of cc-tool in a static
linking scenario on SPARC, i.e to fix:

  http://autobuild.buildroot.net/results/ed9f2524d0ccef318ff1bc99e5dea980111de989/

The patch has been merged upstream, in
https://github.com/dashesy/cc-tool/commit/553f9c601646af0c6e3f6150d0fa3bc63ff56416

.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>