Fix bugs in Cmake setup
Fix bugs in Cmake setup

Summary:
See https://github.com/facebook/folly/issues/1823

* CMAKE_LIBRARY_ARCHITECTURE is not always defined
* This doesn't work: `set(IS_X86_64_ARCH NOT(IS_X86_64_ARCH STREQUAL "-1"))`
* Two conditionals for `IS_X86_64_ARCH` were reversed

Reviewed By: bochko

Differential Revision: D38653631

fbshipit-source-id: c4b6f2820a2280356a7eb69bf0e9253434b5e750

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly