xtensa.sh: fix debug logging

Use correct ${custom_overlay} instead of ${custom_config}

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
samples/xtensa-lx106-elf: disable static libstdc++

Otherwise toolchain build breaks on OS X.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>