build i686 cfg arch

[lmmx]

• build: try with patched scirs
  • rewound build: try with patched scirs #6 to a1903aa
• build: feature-conditional compilation (should still work on x86_64 and i686 but fail on aarch)
  • 2nd attempt to do this after getting known good state merged in Build i686 #5

This doesn't build locally but I expect it will still build on CI. I expect:

• x86_64 will finish first (it will use the scirs2-optimize alpha-2 version from my patch-1 branch)
• i686 will finish in 6 mins (not taking 2 hours because we are using conditional cargo dependencies to keep it on the scirs2-optimize alpha-1 version from my original [merged/contributed] test-powell-quadratic branch)
• aarch64 will (once its config is put in properly) finish fine