unconditionally build symbolizer sources even if FOLLY_USE_SYMBOLIZER is unset or the platform is non-ELF

Summary:
Rather than conditionally include the folly symbolizer sources in the
CMake build, always build them, and add appropriate dependency defines
to conditionally enable functionality.

Reviewed By: yfeldblum

Differential Revision: D22970595

fbshipit-source-id: 957242b7fde28584edd78a1a6dbe9f830e98e726