[XRay] [compiler-rt] Refactor rewinding FDR logging.

Summary:
While there, make the threshold in ticks for the rewind computed only
once and not per function, unify the two versions we had and slightly
reformat bits according to coding standards.

Reviewers: dberris

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31971

llvm-svn: 300028