Various fixes for armv8 and ppc64

With this merge request, it's possible to build minimal-system-content.bst using BuildStream on ppc64b.

It also gets us very close to supporting armv8l64 and ppc64l systems with BuildStream, but the final patches for these will be sent separately as we are still testing.

Most of the changes are fixing issues that were only present in the BuildStream-only bootstrap elements. The old build-essential.morph stratum already supported those architectures and is unchanged.

There are a few component updates in 'core' and 'foundation' too which fix portability issues.