Stabilize Arm64EC inline assembly

[taiki-e]

This stabilizes inline assembly for Arm64EC ("Emulation Compatible").

Corresponding reference PR: rust-lang/reference#1653

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From the requirements of stabilization mentioned in #93335

Each architecture needs to be reviewed before stabilization:

• It must have clobber_abi.

Done in #131332.

• It must be possible to clobber every register that is normally clobbered by a function call.

This is possible from the time of the initial implementation.

• Generally review that the exposed register classes make sense.

The registers available in this target are a subset of those available in the AArch64 inline assembly which is already stable.

The following registers cannot be used in Arm64EC compared to AArch64:

• x13, x14, x23, x24, x28 (register class: reg)
• v[16-31] (register class: vreg)
• p[0-15], ffr (clobber-only register class preg)

These are disallowed by the ABI (see also abi docs for reg/vreg and #131332 (comment) for preg).

Although not listed in the above requirements, preserves_flags is also implemented and the same as AArch64.

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cc @dpaoliello

r? @Amanieu

@rustbot label O-windows O-AArch64 +A-inline-assembly +T-lang -T-compiler +needs-fcp

(T-lang FCP is needed after the review by the reviewer per #131258 (comment))

[dpaoliello]

I'm fine with stabilizing this, but we should include a link to the Microsoft docs: https://learn.microsoft.com/en-us/windows/arm/arm64ec-abi#authoring-arm64ec-in-assembly

At the very least, it should be noted that devs will need to call the appropriate thunks, and I don't believe there's any way to request that new thunks are generated.

[tgross35]

This will probably need lang input like #131258.

@rustbot label +I-lang-easy-decision