folly::Symbolizer can be enhanced for dumpStackTrace

folly::Symbolizer can be enhanced for dumpStackTrace

Summary:
added functionality in Symbolizer.cpp to handle multi-segment binary and to allow symbol lookup via /proc/self/exe when some or whole text is relocated to ANON pages such as during hugification. An example of multi-segment binary : P56071432

cc markw65 - a part of the change include logic pulled from hphp/runtime/base/stack-logger.cpp:symbolize_huge_text so that hugified consumers of dumpStackTrace need not fork off.

Reviewed By: edwardc

Differential Revision: D2802837

fb-gh-sync-id: 577ab1b4ef8f22059894bfdd9c0526a22ee89ca8