Handle NetBSD symbol mangling for nanosleep and vfork

Summary:
Native code generated on NetBSD mangles:

 - vfork into __vfork14
 - nanosleep into __nanosleep50

Handle this accordingly in TSan.

Reviewers: vitalybuka, dvyukov, joerg

Reviewed By: vitalybuka

Subscribers: kubamracek, llvm-commits, #sanitizers, mgorny

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D53806

llvm-svn: 345570