selftests/nolibc: add a "kernel" target to build the kernel with the initramfs

The "kernel" target rebuilds the kernel with the current config for the
selected arch, with an initramfs containing the nolibc-test utility.

Since image names depend on the architecture, the currently supported
ones are referenced and resolved based on the architecture.

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