Auto merge of #147447 - madsmtm:aarch64-watchos-default-deployment-target, r=davidtwco

Set the minimum deployment target for `aarch64-apple-watchos`

To match what's done in LLVM 21 and Xcode 26, watchOS 26 is the first OS version that actually runs true Aarch64 binaries. This affects the object files we create, and the linker invocation when using `-Clinker=ld`.

See also investigation in rust-lang/rust#147223.

Author: bors