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: https://github.com/rust-lang/compiler-team/issues/823
Tracking issue: #135024
Kinda related to the original amdgpu tracking issue #51575 (though that one has been closed for a while).
There are comments saying these two functions should be kept in sync,
but they have very different structures, process symbols in different
orders, and there are some inconsistencies.
This commit reorders them so they're both mostly processing symbols in
alphabetical order, which makes cross-checking them a lot easier. The
commit also adds some macros to factor out repetitive code patterns.
Plus it adds `sanitizer_cfi_normalize_{integers,pointers}` to
`fill_well_known`, which were missing.
The commit also moves the handling of `sym::test` out of
`build_configuration` into `default_configuration`, where all the other
symbols are handled.
