avoid snprintf in Elf code
avoid snprintf in Elf code

Summary: `snprintf` may not be async-signal-safe, but Elf code is intended for use in signal handlers.

Reviewed By: luciang

Differential Revision: D27000080

fbshipit-source-id: 818f48787b5712b7334eca782e18c71c17d03361

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