selftests/nolibc: Avoid generated files being committed

After running the nolibc tests, the "git status" is not clean because
the generated files are not ignored. Create a `.gitignore` inside the
selftests/nolibc directory to ignore them.

Cc: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Cc: Fernanda Ma'rouf <fernandafmr2@gmail.com>
Signed-off-by: Fernanda Ma'rouf <fernandafmr12@gnuweeb.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/testing/selftests/nolibc/.gitignore

+/initramfs/
+/nolibc-test
+/run.out
+/sysroot/