aportgen binutils: modernize
Set CTARGET and CTARGET_ARCH on top of the generated APKBUILD and use
the existing code in the APKBUILD for build(), package(), makedepends,
subpackages. This is now possible, because the pmbootstrap parser for
APKBUILDs has been improved a lot since this was originally written and
because the Alpine APKBUILD does everything that was hacked in here,
like removing man, info directories and bfd-plugins if doing a cross
build.
This is now very similar to bootstrapping binutils for another
architecture via scripts/bootstrap.sh in aports.git.
Most importantly this sets --disable-gprofng, which fails to build
against musl and is the reason why our cross/binutils-* packages
generated from binutils 2.39.x didn't build anymore.