base: replace unimplemented! stubs with explicit panics + cleanup

- ac97d/sb16d: replace non-x86 unimplemented!() with explicit panic
  (these audio buses are x86/x86_64-only by design)
- ps2d: controller + VM stubs — partial work, defer to follow-up
- pcid: cfg_access/fallback stub cleanup
- redoxerd: sys.rs stub cleanup
- bcm2835-sdhcid/ided: partial stub work

All changes replace unimplemented!() macros with either explicit panic
or documented non-support path, eliminating silent panics on non-x86
architectures.

drivers/input/ps2d/src/controller.rs

@@ -129,7 +129,9 @@ impl Ps2 {
 
     #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
     pub fn new() -> Self {
-        unimplemented!()
+        // The PS/2 controller is an x86/x86_64 legacy device. No other
+        // architecture exposes the 0x60/0x64 I/O ports.
+        panic!("ps2d: only supported on x86 and x86_64");
     }
 
     fn status(&mut self) -> StatusFlags {