Add amdgpu_dispatch_ptr intrinsic
There is an ongoing discussion in rust-lang/rust#150452 about using address spaces from the Rust language in some way.
As that discussion will likely not conclude soon, this PR adds one rustc_intrinsic with an addrspacecast to unblock getting basic information like launch and workgroup size and make it possible to implement something like `core::gpu`.
Add a rustc intrinsic `amdgpu_dispatch_ptr` to access the kernel dispatch packet on amdgpu.
The HSA kernel dispatch packet contains important information like the launch size and workgroup size.
The Rust intrinsic lowers to the `llvm.amdgcn.dispatch.ptr` LLVM intrinsic, which returns a `ptr addrspace(4)`, plus an addrspacecast to `addrspace(0)`, so it can be returned as a Rust reference.
The returned pointer/reference is valid for the whole program lifetime, and is therefore `'static`.
The return type of the intrinsic (`&'static ()`) does not mention the struct so that rustc does not need to know the exact struct type. An alternative would be to define the struct as lang item or add a generic argument to the function.
Is this ok or is there a better way (also, should it return a pointer instead of a reference)?
Short version:
```rust
#[cfg(target_arch = "amdgpu")]
pub fn amdgpu_dispatch_ptr() -> *const ();
```
Tracking issue: rust-lang/rust#135024
6912c676cd