selftests/nolibc: "sysroot" target installs a local copy of the sysroot

It's not convenient to rely on a sysroot built in another directory,
especially when running cross-compilation tests, where one has to
switch back and forth between directories.

Let's make it possible to install the sysroot directly in the test
directory. It's not big and even benefits from being copied by arch
so that it's easier to switch between archs if needed. The new
"sysroot" target does this, it just calls "headers_standalone" from
nolibc to install the sysroot right here.

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