Enable VFP support for BeagleBone.
Correct implementation of atomic_flag_test_and_set

The function sets the flag and returns the previous value (7.17.8.1).
Bump up the maximum addressable memory on amd64 systems from 1TB to 4TB.
Bump up the KVA size proportionally from 512GB to 2TB.

The number of page table pages used by the direct map is now calculated at
run time based on 'Maxmem'. This means the small memory systems will not
see any additional tax in terms of page table pages for the direct map.

However all amd64 systems, regardless of the memory size, will use 3 more
pages to accomodate the bump in the KVA size.

More details available here:
http://lists.freebsd.org/pipermail/freebsd-hackers/2013-June/043015.html
http://lists.freebsd.org/pipermail/freebsd-current/2013-July/043143.html

Tested with the following configurations:
- Sandybridge server with 64GB of memory.
- bhyve VM with 64MB of memory.
- bhyve VM with a 8GB of memory with the memory segment above 4GB cuddling
  right up against the 4TB maximum memory limit.

Discussed on:	hackers@, current@
Submitted by:	Chris Torek (torek@torek.net)