Support NotificationQueue on iOS/Android

Support NotificationQueue on iOS/Android

Summary:
This is the last of the changes to make folly and thrift portable. The old
thrift SpinLock implementation is exactly we
want here so port this into folly. If someone later makes efficeint
versions of MicroSpinLock for platforms other than x64 they can easily
be plugged in without changing the calling code.

Test Plan: compiled on multiple platforms

Reviewed By: alandau@fb.com

Subscribers: folly-diffs@, ldbrandy, trunkagent, alandau, bmatheny, njormrod, mshneer, shikong, benyluo, kmdent, fma, ranjeeth, subodh

FB internal diff: D1621717

Tasks: 5183325