Auto merge of #134740 - Flakebi:amdgpu-target, r=workingjubilee

Add amdgpu target

Add amdgpu target to rustc and enable the LLVM target.

Fix compiling `core` with the amdgpu:
The amdgpu backend makes heavy use of different address spaces. This
leads to situations, where a pointer in one addrspace needs to be casted
to a pointer in a different addrspace. `bitcast` is invalid for this
case, `addrspacecast` needs to be used.

Fix compilation failures that created bitcasts for such cases by
creating pointer casts (which creates an `addrspacecast` under the hood)
instead.

MCP: rust-lang/compiler-team#823
Tracking issue: #135024
Kinda related to the original amdgpu tracking issue #51575 (though that one has been closed for a while).

Author: bors