Add option for changing number of GC threads.

The number of threads was previously set to 1 by an accidential
checkin. This hurts on all devices which aren't dual core. We now
properly use sysconf to determine how many threads we should create.

Also added a -XX:HeapGCThreads heap option which lets us change
how many GC threads we create. The default value is equal to the
number of processors on the device minus one.

Change-Id: If65065ef09174a3813b8741efdd5ea7bbe82a4e2