Add the --chroot option for --reproduce.

Summary:
If the linker is invoked with `--chroot /foo` and `/bar/baz.o`, it
tries to read the file from `/foo/bar/baz.o`. This feature is useful
when you are dealing with files created by the --reproduce option.

Reviewers: grimar

Subscribers: llvm-commits, emaste

Differential Revision: https://reviews.llvm.org/D35517

llvm-svn: 308646