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

- Reworked the test as a *centralized* version of checking that certain
  targets correctly require `-C target-cpu` being specified.
- Document test intention.
- Move `amdgpu-require-explicit-cpu.rs` under new dir
  `tests/ui/target-cpu/`
  - No other ui subdir really fits this "requires `-Ctarget-cpu`" check.

tests/ui/amdgpu-require-explicit-cpu.rs

@@ -1,18 +0,0 @@
-//@ revisions: nocpu cpu
-//@ no-prefer-dynamic
-//@ compile-flags: --crate-type=cdylib --target=amdgcn-amd-amdhsa
-//@ needs-llvm-components: amdgpu
-//@ needs-rust-lld
-//@[nocpu] build-fail
-//@[cpu] compile-flags: -Ctarget-cpu=gfx900
-//@[cpu] build-pass
-
-#![feature(no_core, lang_items)]
-#![no_core]
-
-#[lang="sized"]
-trait Sized {}
-
-pub fn foo() {}
-
-//[nocpu]~? ERROR target requires explicitly specifying a cpu with `-C target-cpu`

tests/ui/target-cpu/explicit-target-cpu.avr_nocpu.stderr

@@ -0,0 +1,4 @@
+error: target requires explicitly specifying a cpu with `-C target-cpu`
+
+error: aborting due to 1 previous error
+

tests/ui/target-cpu/explicit-target-cpu.rs

@@ -0,0 +1,37 @@
+//! Check that certain target *requires* the user to specify a target CPU via `-C target-cpu`.
+
+//@ revisions: amdgcn_nocpu amdgcn_cpu
+
+//@[amdgcn_nocpu] compile-flags: --target=amdgcn-amd-amdhsa
+//@[amdgcn_nocpu] needs-llvm-components: amdgpu
+//@[amdgcn_nocpu] build-fail
+
+//@[amdgcn_cpu] compile-flags: --target=amdgcn-amd-amdhsa
+//@[amdgcn_cpu] needs-llvm-components: amdgpu
+//@[amdgcn_cpu] compile-flags: -Ctarget-cpu=gfx900
+//@[amdgcn_cpu] build-pass
+
+//@ revisions: avr_nocpu avr_cpu
+
+//@[avr_nocpu] compile-flags: --target=avr-none
+//@[avr_nocpu] needs-llvm-components: avr
+//@[avr_nocpu] build-fail
+
+//@[avr_cpu] compile-flags: --target=avr-none
+//@[avr_cpu] needs-llvm-components: avr
+//@[avr_cpu] compile-flags: -Ctarget-cpu=atmega328p
+//@[avr_cpu] build-pass
+
+#![crate_type = "rlib"]
+
+// FIXME(#140038): this can't use `minicore` yet because `minicore` doesn't currently propagate the
+// `-C target-cpu` for targets that *require* a `target-cpu` being specified.
+#![feature(no_core, lang_items)]
+#![no_core]
+
+#[lang="sized"]
+trait Sized {}
+
+pub fn foo() {}
+
+//[amdgcn_nocpu,avr_nocpu]~? ERROR target requires explicitly specifying a cpu with `-C target-cpu`