Respect ToolChain::isPIEDefault() in constructing link job on Linux and FreeBSD.

Partially based on http://llvm-reviews.chandlerc.com/D2644 by Viktor Kutuzov.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202150 91177308-0d34-0410-b5e6-96231b3b80d8