[AArch64] Bump default CPUs for iOS 26/watchOS 26 to A12/S6. (#152235)
iOS 26/watchOS 26 deprecate some devices, allowing the default CPU to be
raised based on the deployment target.
watchOS 26 also introduces arm64 (vs. arm64_32/arm64e).
The defaults are now:
- arm64-apple-ios26 apple-a12
- arm64-apple-watchos26 apple-s6
- arm64_32-apple-watchos26 apple-s6
- arm64e-apple-watchos26 apple-s6
Left unchanged are:
- arm64-apple-tvos26 apple-a7
- arm64e-apple-tvos26 apple-a12
- arm64e-apple-ios26 apple-a12
- arm64_32-apple-watchos11 apple-s4
While there, rewrite an outdated comment in a related Mac test.