The MPTable probe code was using PMAP_MAP_LOW as the PA -> VA offset
when searching for the table signature but still using KERNBASE once
it had found the table.  As a result, the mpfps table pointed into a
random part of the kernel text instead of the actual MP Table.

Rather than adding more #ifdef's, use BIOS_PADDRTOVADDR from
<machine/pc/bios.h> which already uses PMAP_MAP_LOW on i386 and KERNBASE
on amd64.

Reviewed by:	kib
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D16802