Runtime detection of available processor cores.

Runtime detection of available processor cores.

Detect the number of available cores and limit the thread allocation
accordingly. On decoder side limit the number of threads to the max
number of token partition.

Core detetction works on Windows and
Posix platforms, which define _SC_NPROCESSORS_ONLN or _SC_NPROC_ONLN.

Change-Id: I76cbe37c18d3b8035e508b7a1795577674efc078