Rather than having a huge list of if-statements that all do the same
thing, this preprocessor allows a more concise pattern to detecting if
the emit-flag is enabled as well as the correlated Xbyak flag that it
needs to check for to before allowing the feature-flag to be emitted.
Also moved the AVX-check to the beginning to early-out rather than do a
bunch of wasted work only to find out last that the host doesn't even
support AVX.
f7c14a089d