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>

tools/testing/selftests/nolibc/run-tests.sh

@@ -143,6 +143,13 @@ test_arch() {
 	arch=$1
 	ct_arch=$(crosstool_arch "$arch")
 	ct_abi=$(crosstool_abi "$1")
+
+	if [ ! -d "${download_location}gcc-${crosstool_version}-nolibc/${ct_arch}-${ct_abi}/bin/." ]; then
+		echo "No toolchain found in ${download_location}gcc-${crosstool_version}-nolibc/${ct_arch}-${ct_abi}."
+		echo "Did you install the toolchains or set the correct arch ? Rerun with -h for help."
+		return 1
+	fi
+
 	cross_compile=$(realpath "${download_location}gcc-${crosstool_version}-nolibc/${ct_arch}-${ct_abi}/bin/${ct_arch}-${ct_abi}-")
 	build_dir="${build_location}/${arch}"
 	if [ "$werror" -ne 0 ]; then