[Driver] [NetBSD] Add -D_REENTRANT when using sanitizers

NetBSD intends to support only reentrant interfaces in interceptors.
When -lpthread is used without _REENTRANT defined, things are
not guaranteed to work.

This is especially important for <stdio.h> and sanitization of
interfaces around FILE.  Some APIs have alternative modes depending
on the _REENTRANT definition, and NetBSD intends to support sanitization
of the _REENTRANT ones.

Differential Revision: https://reviews.llvm.org/D55654

llvm-svn: 349650