Delete vtsc_syscall code

Summary: I can't seem to find any system on which this actually exists. I also
made XHPROF_FLAGS_VTSC behave identically to XHPROF_FLAGS_CPU and
renamed the vtsc() function in this file to avoid confusion. I didn't
remove the PHP-facing flag altogether because some user code still
refers to it.

I still need to figure out if the fallback getrusage() implementation
works well.

Reviewed By: ptarjan, dreiss

Differential Revision: D1481441
Fix sample profiler output; make it more efficient

Summary: Open-source xhprof's sampling profiler outputs an array of
strings mapping to strings. We were outputting strings mapping to
singleton string => int arrays, and the int was always 1. This brings it
in line with open source xhprof.

I also changed the internal storage from a hash table to a vector of
timestamps and stacks, and delayed the conversion of timestamps to
strings until profiling ends. This should lower the profiling overhead,
and has the added bonus that the samples come out in sorted order with
no extra work (open-source xhprof does this too).

Reviewed By: @ptarjan

Differential Revision: D1484272
Document the PHP profilers

Summary: This is everything I've gleaned from studying and modifying
ext_hotprofiler.cpp. I haven't looked at or documented the xdebug
profiler at all, since I'm not sure what state of done-ness it's in.

Reviewed By: @ptarjan

Differential Revision: D1482303