nvptx - thread, grid, and block dim/idx can only return non-negative values

[ZuseZ4]

Based on the previous discussion in https://rust-lang.zulipchat.com/#narrow/channel/422870-t-compiler.2Fgpgpu-backend/topic/Return.20type.20of.20NVPTX.20index.20and.20dimension.20intrinsics

r? @workingjubilee

[rustbot]

stdarch is developed in its own repository. If possible, consider making this change to rust-lang/stdarch instead.

cc @Amanieu, @folkertdev, @sayantn

compiler-builtins is developed in its own repository. If possible, consider making this change to rust-lang/compiler-builtins instead.

cc @tgross35

[rustbot]

workingjubilee is currently at their maximum review capacity. They may take a while to respond.

[workingjubilee]

hm? shouldn't this PR be made against the stdarch repo, since it only affects that subtree? I can accept it, but

[ZuseZ4]

First time contributing to it, sure. New PR: https://github.com/rust-lang/stdarch/pull/1973