These can be manually generated when one really wants to cross compile
from arm to x86_64. Usually this is not needed, so let's save the
compile time. [skip ci]
This covers most use cases and saves a lot of build time. Can be
changed on demand. Again, this simplifies package building as part of
the new build infrastructure effort. [skip ci]
Due to changes in abuild, our `gcc-armhf` etc. packages did not build
when using strict mode (i.e. `pmbootstrap build --strict gcc-armhf`)
anymore.
Changes:
* Set `CBUILDDIR=/`, so apk can read a valid package index from there
* Directly set `_cross_configure`, so it does not use CBUILDDIR anymore
* Set `BOOTSTRAP="nobuildbase"` to prevent apk from installing
`build-base-armhf` etc. (these don't exist in pmOS)
* Remove legacy code for lazy reproducible builds that wrapped
`package()`
Example: Building gcc-armhf for armhf does not make sense, so this
commit changes arch="all" to arch="aarch64 x86_64". This helps to
simplify the repository scripts (#970).
