Change the Profiler constructor to not necessarily force CPU affinity.

Summary: CPU affinity is needed to get monotonic real time clock ticks from rdtsc,
but that effectively disables running multiple threads in server mode.

Some profilers may not need the rdtsc data.

All existing profilers in the code base preserve their previous semantics.
Closes https://github.com/facebook/hhvm/pull/4163

Reviewed By: @JoelMarcey

Differential Revision: D1666121

Signature: t1:1666121:1415832657:60deae3e777ee3bddebd0bf97b0c4bbb3b820486