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/CMakeLists.txt

@@ -5,6 +5,7 @@ set(KERNEL_SOURCES
     ACPI/MultiProcessorParser.cpp
     ACPI/Parser.cpp
     Arch/i386/CPU.cpp
+    Arch/PC/BIOS.cpp
     CMOS.cpp
     CommandLine.cpp
     Console.cpp