define baseline requirements for x86

Historically, Alpine x86 was defined as Intel 80486 CPUs or newer.

However, for a variety of reasons, at present, the x86 architecture assumes the presence of an i586-class CPU with MMX extensions. We should formally document this requirement, as it is leading to occasional bugs such as aports#13016.

Perhaps we should assume the presence of an i686-class CPU at this point? I suspect that there are many applications that are compiled with the assumption that the target CPU is indeed i686-class with SSE2 support, due to autoconf scripts etc detecting support for it on the builder. For example aports#12934 (closed) was caused by building with that kind of assumption.