This reverts commit d56a2facfbc5564cd8fcbc53c47242f7989b3961
and changes the main/gcc6 pkgrel to 9999, so it will always be higher
than Alpine's version.
Based on the last version from Alpine, but installed to /usr/gcc6 and
with pretty much everything disabled except for the C compiler.
We can probably throw out a lot of patches, and make the APKBUILD much
smaller. But let's focus on making it possible to compile old kernels
again first. There's still a lot to do for that, see #103.