Fix Symbios driver on amd64: Since amd64 has 64 bit pointers but the same

Fix Symbios driver on amd64: Since amd64 has 64 bit pointers but the same
4KB pages as i386, data structures that just fit in one page on i386 (and
on 64 bit architectures with 8KB pages) can be distributed over two pages
on amd64. This is a porblem in the case of the Symbios driver, since the
SCRIPTS engine in the SCSI chip operates on physical addresses and needs
physically contiguous memory. Earlier patches used contigmalloc on amd64,
but this version replaces part of a structure by a pointer to that data.
In order to not introduce an extra indirection for other architectures,
the change has been made conditional on __amd64__.

Earlier attempts to repair this problem are removed (i.e. the macros that
made amd64 use contigmalloc). The fix was submitted by Jan Mikkelsen and
modified by me to only affect amd64.

PR:		89550
Submitted by:	janm at transactionware dot com (Jan Mikkelsen)
Approved by:	re (Hiroki Sato)
MFC after:	2 weeks