scripts/bootstrap.sh, binutils, gcc, musl: improve bootstrap

- requires new abuild (from git until 2.30 is tagged)
- properly build initial musl-dev before c-compiler is available
- mark core aports with options=toolchain, and fix all makedepends
- bootstrap.sh: reorganize and use set -e there
- bootstrap.sh: use new EXTRADEPENDS_* to pull in implicit dependencies
  during bootstrap to simplify the script