[Solaris] Only define _REENTRANT if -pthread

When looking at lib/Basic/Targets/OSTargets.h, I noticed that _REENTRANT is defined
unconditionally on Solaris, unlike all other targets and what either Studio cc (only define
it with -mt) or gcc (only define it with -pthread) do.

This patch follows that lead.

Differential Revision: https://reviews.llvm.org/D41241

llvm-svn: 332343