Pull in r184040 from upstream clang trunk:

  Emit native implementations of atomic operations on FreeBSD/armv6.

  Just like on Linux, FreeBSD/armv6 assumes the system supports
  ldrex/strex unconditionally. It is also used by the kernel. We can
  therefore enable support for it, like we do on Linux.

  While there, change one of the unit tests to explicitly test against
  armv5 instead of armv7, as it actually tests whether libcalls are
  emitted.
Move hhook's per-vnet initialisation to an earlier SYSINIT SI_SUB stage to
ensure all per-vnet related hhook initialisation is completed prior to any
virtualised hhook points attempting registration.

vnet_register_sysinit() requires that a stage later than SI_SUB_VNET be chosen.
There are no per-vnet initialisors in the source tree at this time which run
earlier than SI_SUB_INIT_IF. A quick audit of non-virtualised SYSINITs indicates
there are no subsystems pre SI_SUB_MBUF that would likely be interested in
registering a virtualised hhook point.

Settle on SI_SUB_MBUF as hhook's per-vnet initialisation stage as it's the first
overtly network-related initilisation stage to run after SI_SUB_VNET. If a
subsystem that initialises earlier than SI_SUB_MBUF ends up wanting to register
virtualised hhook points in future, hhook's use of SI_SUB_MBUF will need to be
revisited and would probably warrant creating a dedicated SI_SUB_HHOOK which
runs immediately after SI_SUB_VNET.

MFC after:	1 week