Correct NetBSD support in pthread_once(3)/TSan

Summary:
The pthread_once(3)/NetBSD type is built with the following structure:

struct __pthread_once_st {
 pthread_mutex_t pto_mutex;
 int pto_done;
};

Set the pto_done position as shifted by __sanitizer::pthread_mutex_t_sz
from the beginning of the pthread_once struct.

This corrects deadlocks when the pthread_once(3) function
is used.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, dvyukov, vitalybuka

Reviewed By: dvyukov

Subscribers: llvm-commits, kubamracek, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D40262

llvm-svn: 318742