Record time spent in usleep function.

Added 4 counters to Transport to track how long thread sleeps in various sleep functions
when serving a request. I exported the request through hphp_get_timers.

Reviewed By: @mikemag

Differential Revision: D1021385
headers_list should return array() with no headers

headers_list should return an empty array when there are no
 headers. There were already tests involving this in ext_network.php, but it's
 being skipped (at least in the github repository).

Closes #1218

Reviewed By: @alexmalyshev

Differential Revision: D1030259

Pulled By: @scannell
Add sync points in ARM code; sync registers on exceptions from simulated code

Fixups for simulated calls to C++ were basically not working at all.
This fixes a couple of test failures. There are still some failures left
that are failing with a null AR* in fixupWork, which I'm researching
separately.

To add the pseudo-unwind personality to the simulator, I made a
mechanism to provide an "exception hook". Eventually we'll probably want
it to return a flag indicating whether or not to re-throw, but that's
not necessary yet. I made it a hook instead of implementing the logic
right in the catch block to avoid having vixl depend on libruntime.

Reviewed By: @edwinsmith

Differential Revision: D1031833
Fix some bugs with fb_setprofile in ARM mode

- Have separate stubs for the three return helpers. They don't need to
  be implemented yet, but they all need to have distinct addresses
  because we compare against retInlHelper directly to determine whether
  to run function-exit hooks.

- This exposed a bug in cgGuardRefs, where the "is logical immediate"
  assert was firing. This time I actually invested real time into
  understanding what the hell's going on there and rewrote the thing
  properly. (There are still some improvements to be had: most notably,
  when the mask is a single bit, we can use Tst instead of And-Cmp.)

- Fixups done on faked-up ARs in the simulator's catch block were
  succeeding, but not ones done from VMRegAnchors. The underlying cause
  is that the VM stack is not participating in the native frame pointer
  chain, because we're not using native call instructions to get into or
  out of the TC. I added logic to FixupMap that looks at the stack of
  simulators stored in ExecutionContext, and reads their registers to
  look for the place to apply the fixup.

Reviewed By: @edwinsmith

Differential Revision: D1032483
Use gcc-4.8 on travis for faster builds