Re-enabling ACPI module.

[Takanori Watanabe]

Hi, I looked in the new APIC code you commit.
The thing that prevent ACPI subsystem from moduler 
is the way we aquire ACPI root pointer, I think. Right?

If so, we can took this in  way.
1. Move 

u_long i386_acpi_root;
to machdep.c and madt.c use i386_acpi_root variable, instead of calling
AcpiOsGetRootPointer. If Boot loader failed to detect ACPI root pointer,
the MADT table is not used, even if the OS can detect it.

2. Re-imprement AcpiOsGetRootPointer so that it does not 
use AcpiFindRootPointer().

Which do you think better?