[XRay][compiler-rt] Profiling Mode: Flush logs on exit

Summary:
This change adds support for writing out profiles at program exit.

Depends on D48653.

Reviewers: kpw, eizan

Reviewed By: kpw

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D48956

llvm-svn: 336969