Support libdwarf prior to 5
Support libdwarf prior to 5

Summary:
Not all systems have a libdwarf that supports dwarf 5. We only need a single tag from it, so just define it when needed.
Fixes: https://github.com/facebook/folly/issues/1654

Reviewed By: luciang

Differential Revision: D31423305

fbshipit-source-id: 89fecccb5ac68830a33b1fad51210cbaa9754bf8

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