Fix incorrect 'llvm_target' value used on watchOS target

[crazytonyli]

Issue

xcodebuild -create-xcframework command doesn't recognize static libraries that are built on "arm64_32-apple-watchos" target.

Here are steps to reproduce the issue on a Mac:

1. Install nightly toolchain nightly-2024-03-27. Needs this specific version, because newer nightly versions are broken on watchos target.
2. Create an empty library: mkdir watchos-lib && cd watchos-lib && cargo init --lib.
3. Add configuration lib.crate-type=["staticlib"] to Cargo.toml.
4. Build the library: cargo +nightly-2024-03-27 build --release -Zbuild-std --target arm64_32-apple-watchos
5. Run xcodebuild -create-xcframework to put the static library into a xcframework, which results in an error:

$ xcodebuild -create-xcframework -library target/arm64_32-apple-watchos/release/libwatchos_lib.a -output test.xcframework
error: unable to determine the platform for the given binary '.../watchos-lib/target/arm64_32-apple-watchos/release/libwatchos_lib.a'; check your deployment version settings

Fix

The root cause of this error is xcodebuild couldn't read LC_BUILD_VERSION from the static library to determine the library's target platform. And the reason it's missing is that an incorrect llvm_target value is used in arm64_32-apple-watchos target. The expected value is <arch>-apple-watchos<major>.<minor>.0, i.e. "arm64_32-apple-watchos8.0.0".

The .../apple/mod.rs file contains functions that construct such string. There is an existing function watchos_sim_llvm_target which returns llvm target value for watchOS simulator. But there is none for watchOS device. This PR adds that missing function to align watchOS with other Apple platform targets.

To verify the fix, you can simply build a toolchain on this PR branch and repeat the steps above using the built local toolchain to verify the xcodebuild -create-xcframework command can create a xcframework successfully.

Furthermore, you can verify LC_BUILD_VERSION contains correct info by using the simple shell script below to print LC_BUILD_VERSION of the static library that's built on watchos target:

bin=target/arm64_32-apple-watchos/release/libwatchos_lib.a
file=$(ar -t "$bin" | grep -E '\.o$' | head -n 1)
ar -x "$bin" "$file"
vtool -show-build-version "$file"

Here is an example output from my machine:

watchos_rust-495d6aaf3bccc08d.watchos_rust.35ba42bf9255ca9d-cgu.0.rcgu.o:
Load command 1
      cmd LC_BUILD_VERSION
  cmdsize 24
 platform WATCHOS
    minos 8.0
      sdk n/a
   ntools 0

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @estebank (or someone else) some time within the next two weeks.

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[rustbot]

These commits modify compiler targets.
(See the Target Tier Policy.)

[estebank]

cc @rust-lang/compiler for changes to watch OS target

The changes look reasonable.

@bors r+

[bors]

📌 Commit 1ffb410 has been approved by estebank

It is now in the queue for this repository.