Kernel: Add convenient ways to map whole BIOS and EBDA into memory

This patch adds a MappedROM abstraction to the Kernel VM subsystem.
It's basically the read-only byte buffer equivalent of a TypedMapping.

We use this in the ACPI and MP table parsers to scan for interesting
stuff in low memory instead of doing a bunch of address arithmetic.

Kernel/Forward.h

@@ -44,6 +44,7 @@ class InodeWatcher;
 class KBuffer;
 class KResult;
 class LocalSocket;
+class MappedROM;
 class PageDirectory;
 class PerformanceEventBuffer;
 class PhysicalPage;