tools/nolibc: add the nolibc subdir to the common Makefile

The Makefile in tools/ is used to forward options to the makefiles
in the various subdirs. Let's add nolibc there so that it becomes
possible to make tools/nolibc_headers_standalone from the main tree
to simply create a completely usable sysroot.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>