Adapt NVPTX target-feature handling for LLVM 22

[kulst]

Previously, we did not explicitly enable a PTX version target-feature, because LLVM implied one from the selected CPU.

Starting with LLVM 22, the NVPTX backend still infers the minimum required PTX version from the selected CPU when none is selected explicitly, but no longer reflects that version in the enabled subtarget features. As a result, cfg could miss the PTX version of the emitted PTX. Also, if a PTX version below the required minimum is selected explicitly, LLVM errors.
We missed this because the corresponding NVPTX target-feature test was mistakenly disabled.

With this patch we always select a PTX version explicitly, whichever is higher between

• Rust's minimum required PTX version, and
• the CPU's minimum required PTX version.

The patch also re-enables the NVPTX target-feature test to catch such changes in the future.
It additionally forbids PTX and sm versions below Rust's minimum required versions by marking the corresponding target-features as Forbidden.
Lastly it adds PTX and sm versions present in LLVM 22 but missing as Rust target-features.

This will not prevent all issues. In fact, the following combinations might still miscompile:

-Ctarget-cpu=sm_101 -Ctarget-feature=+ptx90 // sm_101 was renamed starting with ptx90
-Ctarget-cpu=sm_70 -Ctarget-feature=+sm_80,+ptx81 // .target directive is sm_70 but instructions are selected from sm_80

cc: @kjetilkjeka
r? @ZuseZ4
@rustbot label +O-NVPTX