selftests/nolibc: run-tests.sh: detect missing toolchain

The script tries to resolve the path to the current toolchain using
realpath, which fails in case it's not installed, and since it's run
under -e, it doesn't have the opportunity to display a help message.
Let's detect the absence of the required toolchain before running that
command and provide a friendlier message when this happens.

Link: https://lore.kernel.org/all/ZtlQbpgpn9OQOPyI@1wt.eu/


Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>