Add support for Arm64EC inline assembly (as unstable)

[dpaoliello]

Compiler support for Arm64EC assembly mostly reuses the existing AArch64 support, except that it needs to block registers that are not permitted: https://learn.microsoft.com/en-us/windows/arm/arm64ec-abi#register-mapping-and-blocked-registers

For assembly authors there are several caveats and differences that need to be considered, I've provided documentation for this as part of the "Standard Library Support" PR: https://github.com/rust-lang/rust/pull/123144/files#diff-6b08532480943c8b82f5dbda7ee1521afa74c9f626466aeb308dfa6956397edd

r? rust-lang/compiler

[BoxyUwU]

r? @Amanieu

[Amanieu]

@bors r+

[bors]

📌 Commit 2e44d29 has been approved by Amanieu

It is now in the queue for this repository.

[bors]

⌛ Testing commit 2e44d29 with merge 05ccc49...

[bors]

☀️ Test successful - checks-actions
Approved by: Amanieu
Pushing 05ccc49 to master...

[rust-timer]

Finished benchmarking commit (05ccc49): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.2%	[-1.2%, -1.2%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.2%	[-1.2%, -1.2%]	1

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.4%	[-1.4%, -1.4%]	1
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 676.139s -> 675.139s (-0.15%)
Artifact size: 316.07 MiB -> 315.95 MiB (-0.04%)