Add support for background thread on NetBSD in ASan

Summary:
Change the point of calling MaybeStartBackgroudThread() from AsanInitInternal()
that is too early on NetBSD to a constructor (with aid of C++11 lambda construct).

Enable the code for background thread as is for NetBSD.

Rename test/sanitizer_common/TestCases/Linux/hard_rss_limit_mb_test.cc
to test/sanitizer_common/TestCases/hard_rss_limit_mb_test.cc and allow runs
on NetBSD. This tests passes correctly.

Reviewers: vitalybuka, joerg, eugenis

Reviewed By: eugenis

Subscribers: eugenis, kubamracek, fedor.sergeev, llvm-commits, mgorny, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D55887

llvm-svn: 350139