Allow target specs to use an LLD flavor, and self-contained linking components

[lqd]

This PR allows:

• target specs to use an LLD linker-flavor: this is needed to switch x86_64-unknown-linux-gnu to using LLD, and is currently not possible because the current flavor json serialization fails to roundtrip on the modern linker-flavors. This can e.g. be seen in Enable lld explicitly for hexagon-unknown-linux-musl #115622 (comment) which explains where an Lld::Yes is ultimately deserialized into an Lld::No.
• target specs to declare self-contained linking components: this is needed to switch x86_64-unknown-linux-gnu to using rust-lld
• adds an end-to-end test of a custom target json simulating x86_64-unknown-linux-gnu being switched to using rust-lld
• disables codegen backends from participating because they don't support -Zgcc-ld=lld which is the basis of mcp510.

r? @petrochenkov: if the approach discussed #115622 (comment) and on zulip would work for you: basically, see if we can emit only modern linker flavors in the json specs, but accept both old and new flavors while reading them, to fix the roundtrip issue.

The backwards compatible LinkSelfContainedDefault variants are still serialized and deserialized in crt-objects-fallback, while the spec equivalent of e.g. -Clink-self-contained=+linker is serialized into a different json object (with future-proofing to incorporate crt-objects-fallback in the future).

---

I've been test-driving this in #113382 to test actually switching x86_64-unknown-linux-gnu to rust-lld (and fix what needs to be fixed in CI, bootstrap, etc), and it seems to work fine.

[rustbot]

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

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

[petrochenkov]

Blocked on #116014.
@rustbot blocked

[lqd]

Rebased to fix conflicts.

[lqd]

Rebased now that #11601 has now landed and unblocked this PR.

@rustbot ready.

[petrochenkov]

@bors r+

[bors]

📌 Commit 0b40c7c has been approved by petrochenkov

It is now in the queue for this repository.

[bors]

⌛ Testing commit 0b40c7c with merge 31ffe48...

[bors]

☀️ Test successful - checks-actions
Approved by: petrochenkov
Pushing 31ffe48 to master...

[rust-timer]

Finished benchmarking commit (31ffe48): 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)	2.9%	[2.0%, 3.6%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
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)	10.0%	[9.9%, 10.1%]	2
Improvements ✅ (primary)	-1.3%	[-1.3%, -1.3%]	1
Improvements ✅ (secondary)	-2.3%	[-2.3%, -2.3%]	1
All ❌✅ (primary)	-1.3%	[-1.3%, -1.3%]	1

Binary size

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

Bootstrap: 636.976s -> 635.259s (-0.27%)
Artifact size: 304.49 MiB -> 304.44 MiB (-0.02%)