RFC: implement optimization profiles

Presently, abuild only supports a unified set of CFLAGS, et al. If you want to override the CFLAGS used for a package, you must do it in the APKBUILD for that package. However, there are many packages where the default -Os optimization setting does not make any sense: for the amount of binary size savings, we lose significant performance -- an example of this being the OpenBLAS and LAPACK libraries.

Talking about this with @jirutka we thought about the concept of an optimization profile: a package maintainer would specify how the build should be optimized, and abuild would choose the correct CFLAGS based on that profile. For example, an APKBUILD could have optprofile=size or optprofile=performance, which would choose different CFLAGS based on the requested profile. It would be desirable for the end-user to be able to override the default profiles in abuild.conf, as well.

Thinking out loud, there could be at least three profiles, but we may want additional profiles to trade hardening features for performance too:

• size: the current default CFLAGS: -Os -fomit-frame-pointer
• balanced: basically the same as the current default, but -O2
• performance: -O3 -ffast-math -funroll-all-loops ...

We probably want to tweak these profiles for specific microarchitectural optimizations, too, but that seems like something where @alxu probably knows a lot more than me.

The point of this being that we no longer are just overriding CFLAGS to get -O2 and so on, it keeps everything more consistent with the building.