Add missing s6_addr16 define for MacOSX targets

Summary:
This is a known issue on Darwin

@override-unit-failures

Test Plan: folly/detail/IPAddress.h compiles on a mac

Reviewed By: joelm@fb.com

FB internal diff: D1499795
don't use readFromSysfs on non-linux

Summary:
It eventually tries to read from /sys/devices which is not portable.
Let's just fall back to sysconf/CacheLocality::uniform.

This fixes a runtime crash on OSX, but CYGWIN/BSD were probably also
affected.
Closes #80

@override-unit-failures

Test Plan: author tested, contbuild, flib

Reviewed By: joelm@fb.com

FB internal diff: D1498567
Bump version to 3:2