Don't generate listing by default, remove unused LIBGCC variable and
rename generated disassembly and listing files to vmlinux.{s,lst}.

Also make sure that files generated during the build are actually
removed with make clean.
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>