[AArch64] Don't always override CPU for arm64e.

This demotes the apple-a12 CPU selection for arm64e to just be the
last-resort default.  Concretely, this means:
- an explicitly-specified -mcpu will override the arm64e default;
  a user could potentially pick an invalid CPU that doesn't have
  v8.3a support, but that's not a major problem anymore
- arm64e-apple-macos (and variants) will pick apple-m1 instead of
  being forced to apple-a12.