x86: make the arrays that depend on MAX_APIC_ID dynamic

So that MAX_APIC_ID can be bumped without wasting memory.

Note that the usage of MAX_APIC_ID in the SRAT parsing forces the
parser to allocate memory directly from the phys_avail physical memory
array, which is not the best approach probably, but I haven't found
any other way to allocate memory so early in boot. This memory is not
returned to the system afterwards, but at least it's sized according
to the maximum APIC ID found in the MADT table.

Sponsored by: Citrix Systems R&D
MFC after: 1 month
Reviewed by: kib
Differential revision: https://reviews.freebsd.org/D11912