Add Arm64_32 variant to Aarch64Architecture for arm64_32-apple-watchos

[matthargett]

Summary

Adds a third Arm64_32 variant to Aarch64Architecture so the
arm64_32-apple-watchos Rust target triple round-trips through
Triple::from_str / Display.

Apple Watch Series 4+ ships an AArch64 ISA with an ILP32 ABI: 64-bit
registers, 32-bit pointers. Apple/LLVM treat the arch token as
arm64_32 (not as a gnu_ilp32-style environment qualifier the way
Linux does). The current crate exposes only Aarch64 and Aarch64be,
both hardcoded to pointer_width = U64, so any consumer that calls
Triple::from_str("arm64_32-apple-watchos") panics with
Invalid target name. There is a // TODO placeholder for this triple
in the roundtrip_known_triples test that this PR un-comments.

Why this matters

Without this patch nothing in the bytecodealliance ecosystem can build
for arm64_32-apple-watchos — wasmtime / cranelift consume
target-lexicon transitively through wasmtime-environ and others,
and their build scripts call Triple::from_str(env!(\"TARGET\")). The
panic surfaces during cargo build --target arm64_32-apple-watchos
before any target-side code is even attempted.

This is the same pattern as the existing aarch64-apple-watchos /
aarch64-apple-watchos-sim entries — those are aarch64 with U64
pointers; arm64_32 is the same ISA with U32 pointers.

What changed

1. New variant Aarch64Architecture::Arm64_32 (with a doc comment
   explaining the Apple/LLVM naming convention).
2. pointer_width() returns U32 for the new variant, U64 for the
   existing two — preserving the doc note that this method is
   normally arch-only and ILP32-blind, with an explicit carve-out for
   arm64_32 (which Apple models as its own arch, not via environment).
3. endianness() returns Little.
4. into_str() returns \"arm64_32\" so Display / to_string()
   round-trip the original token.
5. is_thumb() returns false (consistent with the other AArch64
   variants).
6. Aarch64Architecture::FromStr accepts \"arm64_32\".
7. The known-triples test list at `src/targets.rs` un-comments the
   \"arm64_32-apple-watchos\" entry that was already there as a
   // TODO placeholder.

Test

cargo test — all 14 tests pass, including
roundtrip_known_triples which exercises the new triple.

Verified end-to-end on real hardware

This patch is one of three needed to build wasmtime+Pulley as a static
library for arm64_32-apple-watchos. The combined stack has been
deployed to a real Apple Watch SE 2 (S8 SoC, watchOS 11.6.2), where
all 11 wasm benchmarks (recursive fib, tail-call fib, sieve, CRC32,
two SIMD matmuls, convolution, audio-DSP, bulk-memory, call_indirect)
run with byte-identical results to their host references.

[alexcrichton]

cc @fitzgen, would you be able to review/merge this?

[fitzgen]

LGTM, thanks!