stable/21.x: [AArch64] Bump default CPUs for iOS 18/26, watchOS 26 to A10/A12/S6/S9.

[ahmedbougacha]

[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While there, rewrite an outdated comment in a related Mac test.

(cherry picked from commit 69d141712a107ed9f38fbfa12b0c35504f8bfb7c)


[AArch64] Bump default CPUs for iOS 18/watchOS 26 to A10/S9. (#162325)

We previously bumped the defaults for 26.0 Apple OS targets to
conservative CPU targets in 69d141712a10.

We can further bump it for watchOS arm64 (which lets us be a little bit
more aggressive than arm64e/arm64_32), as well as for the iOS
predecessor, iOS 18.

(cherry picked from commit b54f01e91163abb7cfcca76da34efebbee4622b4)

[ahmedbougacha]

@swift-ci test

[ahmedbougacha]

@swift-ci test macOS platform