Add testing for Arm64EC Windows

[dpaoliello]

• Add a CI job for Arm64EC Windows on the new Arm64 Windows 11 runners.
• Removes the dedicated step to install rustup on the Arm64 Windows 11 runners as it's not included.
• Disable FP 16 tests, as it's not supported for Arm64EC.
• Fix disassembling Arm64EC code.

[dpaoliello]

Had a clean build here: https://github.com/rust-lang/stdarch/actions/runs/16865911732/job/47772786509?pr=1899

Looks like s390x is currently broken in CI.

[rustbot]

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[folkertdev]

Right, we're blocked on #1897. The fix for our issue has been merged and cherry-picked into the next LLVM point release, but we'll have to wait a bit I guess before rust adopts that and we can use it.

[folkertdev]

For context, is the lack of fp16 support something fundamental, or is it a temporary limitation (e.g. in msvc)?

[tgross35]

For context, is the lack of fp16 support something fundamental, or is it a temporary limitation (e.g. in msvc)?

Currently it's a crash in LLVM and MSVC doesn't support it. I'm hoping that llvm/llvm-project#152843 is all that will be needed to make the scalar type work (if so, we can probably consider stabilizing it after we get LLVM22). If so, AIUI there shouldn't be any problem with these intrinsics.