Add checks for gpu-kernel calling conv
The `gpu-kernel` calling convention has several restrictions that were not enforced by the compiler until now.
Add the following restrictions:
1. Cannot be async
2. Cannot be called
3. Cannot return values, return type must be `()` or `!`
4. Arguments should be simple, i.e. passed by value. More complicated types can work when you know what you are doing, but it is rather unintuitive, one needs to know ABI/compiler internals.
5. Export name should be unmangled, either through `no_mangle` or `export_name`. Kernels are searched by name on the CPU side, having a mangled name makes it hard to find and probably almost always unintentional.
Tracking issue: rust-lang/rust#135467
amdgpu target tracking issue: rust-lang/rust#135024
``@workingjubilee,`` these should be all the restrictions we talked about a year ago.
cc ``@RDambrosio016`` ``@kjetilkjeka`` for nvptx
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