Add a FIXME comment to the inline(never) and link to the GitHub issue

Author: phil-opp

Diff for: bios/stage-4/src/main.rs

@@ -262,6 +262,9 @@ fn detect_rsdp() -> Option<PhysAddr> {
     #[derive(Clone)]
     struct IdentityMapped;
     impl AcpiHandler for IdentityMapped {
+        // TODO FIXME: This inline(never) annotation is required. Without it,
+        // LLVM replaces the `search_for_on_bios` call below with a `ud2`
+        // instruction. See https://github.com/rust-osdev/bootloader/issues/425
         #[inline(never)]
         unsafe fn map_physical_region<T>(
             &self,