Add self-profile events for target-machine creation

[Zalathar]

These code paths are surprisingly hot in the large-workspace benchmark (e.g. see perf changes from #146700), suggesting room for more improvement. It would be handy to see some detailed timings and execution counts.

[rustbot]

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[Zalathar]

This PR won't have a perf impact on its own, but I would like to see a preview of the measurements it produces.

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 532db74 (532db7469e9a92676bcc8a3a65102314490c6df3, parent: 1d23da6b7304d9e2a2c3dcb1b0aaa709cb9bc4ad)

[rust-timer]

Finished benchmarking commit (532db74): comparison URL.

Overall result: ❌ regressions - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.6%	[0.6%, 0.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 3.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	3.5%	[3.5%, 3.5%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.5%	[3.5%, 3.5%]	1

Cycles

Results (primary 5.0%, secondary -0.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	5.0%	[5.0%, 5.0%]	1
Regressions ❌ (secondary)	4.6%	[4.6%, 4.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.6%	[-2.9%, -2.3%]	2
All ❌✅ (primary)	5.0%	[5.0%, 5.0%]	1

Binary size

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

Bootstrap: 470.725s -> 470.112s (-0.13%)
Artifact size: 387.92 MiB -> 387.90 MiB (-0.01%)