suggest xtoolchain package if binutils and GCC bootstraps are both broken

Previously we checked for only BINUTILS_BOOTSTRAP as a broken option
and suggested installing the binutils package. This was originally done
for arm64 where we used the in-tree Clang with external binutils package.

Add a case to the warning to suggest instead the full xtoolchain package
if we have no in-tree compiler either.

Differential Revision: https://reviews.freebsd.org/D21851