Fix the MPTable probe code after the 4:4 changes on i386.

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