Bump WITNESS_PENDLIST by MAXCPU to account for the
pmap pvlist locks which are scaled by MAXCPU.

This allows an amd64 system to boot with MAXCPU set
to 256, which is currently FreeBSD's hard limit without
x2apic support.

Compile-tested for other arch's.

PR:	185831
Discussed with:		jhb
MFC after:	3 weeks