tests: adjust amdgpu-require-explicit-cpu.rs

- Document test intention.
- Move `amdgpu-require-explicit-cpu.rs` under new dir `tests/ui/amdgpu/`
  - No other ui subdir really fits this "requires `-Ctarget-cpu`" check.
  - I don't think any other targets require explicitly specifying a
    `-Ctarget-cpu` in addition to `--target` itself.

Author: jieyouxu

Diff for: tests/ui/amdgpu-require-explicit-cpu.nocpu.stderr renamed to tests/ui/amdgcn/amdgpu-require-explicit-cpu.nocpu.stderr

@@ -1,3 +1,6 @@
+//! Check that compiling against the `amdgcn-amd-amdhsa` target *requires* the user to specify a
+//! target CPU via `-C target-cpu`.
+
 //@ revisions: nocpu cpu
 //@ no-prefer-dynamic
 //@ compile-flags: --crate-type=cdylib --target=amdgcn-amd-amdhsa