ci: upgrade and refactor crosstool-ng builders

[cuviper]

The first commit upgrades our builders from crosstool-ng 1.24.0 to 1.25.0. There are otherwise no changes intended to the toolchains we're using, but there are some minor version upgrades as a result, especially GCC 8.3.0 to 8.5.0. The newer crosstool-ng will position us well to make toolchain upgrades in the future though, as we were maxed out before and it now goes up to GCC 11.

The second commit refactors our config management to only commit a "mini-defconfig" for each target, produced by ct-ng savedefconfig. This makes it much clearer which settings we're actually changing, and also makes it easier to ensure consistency for things like mirror management.

[rustbot]

r? @pietroalbini

(rustbot has picked a reviewer for you, use r? to override)

[pietroalbini]

Awesome, wanted something like this to happen for so long ❤️

@bors r+ 🎉

[bors]

📌 Commit 16dfd89 has been approved by pietroalbini

It is now in the queue for this repository.

[pietroalbini]

@bors rollup=never

[bors]

⌛ Testing commit 16dfd89 with merge 9b99ff7...

[bors]

☀️ Test successful - checks-actions
Approved by: pietroalbini
Pushing 9b99ff7 to master...

[rust-timer]

Finished benchmarking commit (9b99ff7): 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)	2.4%	[2.4%, 2.5%]	2
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)	-	-	0
Improvements ✅ (primary)	-1.5%	[-1.5%, -1.5%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.5%	[-1.5%, -1.5%]	1

Binary size

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

Bootstrap: 654.411s -> 654.621s (0.03%)