extend in-tree toolchain check to suggest xtoolchain package

[emaste]

Summary

If binutils/lld and GCC bootstraps are broken and we're not using Clang, suggest installing the xtoolchain-gcc package instead of just binutils.

[emaste]

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 and external binutils package. Extend the warning to suggest instead the full xtoolchain package if we have no in-tree compiler either.