tcmode: improve the msgxx-glibc check

All the toolchains I've tested don't actually show an error message for an
unrecognized -m argument when using -print-sysroot. We need to actually try to
compile something, so give it an empty file.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
external-run: use a warning rather than error, add missing import

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Extract PV/LICENSE immediately

Signed-off-by: Christopher Larson <chris_larson@mentor.com>