Add config option for experimental LLVM targets

[tlively]

NVPTX and Hexagon LLVM targets are no longer built by default. Instead,
the developer has to opt in to building them by listing them in the new
experimental-targets option.

This change was suggested in #42571.
cc @eholk
r? @brson

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[tlively]

This change is breaking for users of Hexagon and NVPTX, since they would have to update their config.toml. It also only works because LLVM's CMake setup does not differentiate between LLVM_TARGETS_TO_BUILD and LLVM_EXPERIMENTAL_TARGETS_TO_BUILD as far as I can tell. However, given that, this change may not be necessary, since the WebAssembly target in #42571 can simply be optionally added to the normal list of targets.

[hanna-kruppe]

This change is breaking for users of Hexagon and NVPTX, since they would have to update their config.toml

They would have to start building rustc themselves, if they don't already (there's no reason for them to do so currently).

I'd like to hear some motivation for disabling these targets. The binary size increases were measured at least in the Hexagon case, and found to be neglegible. The build time shouldn't be a large factor either.

[tlively]

An alternative to this change (assuming removing NVPTX and Hexagon default support is still desired) would be to simply remove NVPTX and Hexagon from the defaults for the targets option. The only annoying thing about that would be that in order to build for the default targets in addition of Hexagon, for example, the user would have to manually list all the default targets and Hexagon instead of having to add only Hexagon.

[hanna-kruppe]

Isn't the config.toml example supposed to contain a (commented out) up-to-date list of the default targets?

[tlively]

Isn't the config.toml example supposed to contain a (commented out) up-to-date list of the default targets?

Do you mean src/bootstrap/config.toml.example? This PR does update that file to reflect what would be the new defaults.

[hanna-kruppe]

So if that file contains the default list of targets, I don't see how this is a burden at all:

The only annoying thing about that would be that in order to build for the default targets in addition of Hexagon, for example, the user would have to manually list all the default targets and Hexagon instead of having to add only Hexagon.

[tlively]

You're right, on second thought that's fairly trivial.

It seems to me that the best option would be to have our definition of "experimental" exactly match that of LLVM. That would mean the NVPTX and Hexagon would remain built by default, and this new configuration option would only be used for WebAssembly right now. In that case, this change can be made directly in #42571 and this PR can be closed.

@rkruppe, does that sound reasonable to you?

[hanna-kruppe]

So the difference to the status quo would be that there's a way to pass -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD to CMake, and nothing else as far as non-wasm targets are concerned? Yes, that sounds good.