Driver: implement --dyld-prefix option.

This option is used to select a dynamic loader prefix to be used
at runtime. Currently this is implemented for the Linux toolchain.

Differential Revision: http://llvm-reviews.chandlerc.com/D851

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182744 91177308-0d34-0410-b5e6-96231b3b80d8