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.
apple-m1 has the same level of ISA support as apple-a14,
so this is a straightforward mechanical change. However, that
also means this inherits apple-a14's v8.5a+nobti quirkiness.
rdar://68287159
Ahmed Bougacha