split SmallLocks.h to get a larger portion of folly compiled on 32bit platforms

split SmallLocks.h to get a larger portion of folly compiled on 32bit platforms

Summary: This is only a simple #include file split, no functional change was made. SmallLocks.h provides 2 classes: MicroSpinLock and PicoSpinLock, with the latter not being available on 32bit platforms. There are several classes in folly that included SmallLocks.h but only needed MicroSpinLock, these also refused to compile on 32bit, e.g. folly/futures/Future.h. Now these compile fine.

Reviewed By: @yfeldblum

Differential Revision: D2234263