Enable SafeStack on NetBSD

Summary:
make check-safestack:

-- Testing: 8 tests, 8 threads --
Testing: 0 .. 10.. 20.. 30.. 40.. 50.. 60.. 70.. 80.. 90..
Testing Time: 0.44s
  Expected Passes    : 7
  Unsupported Tests  : 1

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, vitalybuka, kcc, fjricci, filcab

Reviewed By: vitalybuka

Subscribers: mgorny, llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D36542

llvm-svn: 310646