Defer StartBackgroundThread() and StopBackgroundThread() in TSan

Summary:
NetBSD cannot spawn new POSIX thread entities in early
libc and libpthread initialization stage. Defer this to the point
of intercepting the first pthread_create(3) call.

This is the last change that makes Thread Sanitizer functional
on NetBSD/amd64 without downstream patches.

********************
Testing Time: 64.91s
********************
Failing Tests (5):
    ThreadSanitizer-x86_64 :: dtls.c
    ThreadSanitizer-x86_64 :: ignore_lib5.cc
    ThreadSanitizer-x86_64 :: ignored-interceptors-mmap.cc
    ThreadSanitizer-x86_64 :: mutex_lock_destroyed.cc
    ThreadSanitizer-x86_64 :: vfork.cc

  Expected Passes    : 290
  Expected Failures  : 1
  Unsupported Tests  : 83
  Unexpected Failures: 5

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, eugenis, dvyukov, vitalybuka

Reviewed By: dvyukov

Subscribers: kubamracek, llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D40583

llvm-svn: 319305