Add support for LLVM SafeStack

[wesleywiser]

Adds support for LLVM SafeStack which provides backward edge control
flow protection by separating the stack into two parts: data which is
only accessed in provable safe ways is allocated on the normal stack
(the "safe stack") and all other data is placed in a separate allocation
(the "unsafe stack").

SafeStack support is enabled by passing -Zsanitizer=safestack.

cc @rcvalle #39699

[rustbot]

r? @GuillaumeGomez

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[rustbot]

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

[rcvalle]

LGTM. Thank you for your time and for working on this, @wesleywiser! Much appreciated.

[GuillaumeGomez]

r? @Amanieu

[Amanieu]

@bors r+

[bors]

📌 Commit 019d75b has been approved by Amanieu

It is now in the queue for this repository.

[bors]

⌛ Testing commit 019d75b with merge 2560b80...

[bors]

☀️ Test successful - checks-actions
Approved by: Amanieu
Pushing 2560b80 to master...

[rust-timer]

Finished benchmarking commit (2560b80): 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)	1.0%	[1.0%, 1.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.9%	[-1.9%, -1.9%]	1
All ❌✅ (primary)	-	-	0

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)	-1.3%	[-1.3%, -1.3%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 645.767s -> 644.189s (-0.24%)