Add gpu-kernel calling convention The amdgpu-kernel calling convention was reverted in commit f6b21e90d1ec01081bc2619efb68af6788a63d65 (#120495 and https://github.com/rust-lang/rust-analyzer/pull/16463) due to inactivity in the amdgpu target. Introduce a `gpu-kernel` calling convention that translates to `ptx_kernel` or `amdgpu_kernel`, depending on the target that rust compiles for. Tracking issue: #135467 amdgpu target tracking issue: #135024