Tolerate flaky LLD tests on NetBSD

Summary:
For unknown reasons  LLD tests are flaky on the NetBSD buildbot,
but not on local machines of developers.

Unless the linker will be fully functional on this target,
allow to pass flaky tests with optional retry.

Reviewers: joerg, mgorny, ruiu

Reviewed By: mgorny

Subscribers: lebedev.ri, MaskRay, llvm-commits, #lld

Tags: #lld

Differential Revision: https://reviews.llvm.org/D56053

llvm-svn: 350036