Add support for microarch features levels

GCC 11 and Clang 12 introduce support for micro-architecture levels defined in the x86-64 psABI (via -march=x86-64-v2, -march=x86-64-v3 and -march=x86-64-v4), see GCC 11 release notes.

The definition of the levels is simple enough that it can be easily mimicked on older versions of the compilers, and that is what I'm doing with this MR, similar to lcg-toolchains!47 (2eac0d17)