Rollup merge of #138689 - jedbrown:jed/nvptx-target-feature, r=ZuseZ4

add nvptx_target_feature

Tracking issue: #141468 (nvptx), which is part of #44839 (catch-all arches)
The feature gate is `#![feature(nvptx_target_feature)]`

This exposes the target features `sm_20` through `sm_120a` [as defined](https://github.com/llvm/llvm-project/blob/llvmorg-20.1.1/llvm/lib/Target/NVPTX/NVPTX.td#L59-L85) by LLVM.

Cc: ``````@gonzalobg``````
``````@rustbot`````` label +O-NVPTX +A-target-feature

Author: Zalathar