[lsan] When necessary, define LSan suppression for pthread_exit.

Summary:
Generalize already defined LSan suppression for the leak on
pthread_exit, some envs do not have the entire call stack symbolized,
so we have to be less specific.

Reviewers: eugenis

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32497

llvm-svn: 301335