[ELF] Fix rewriting of --sysroot in reproducer tars

Summary:
I noticed that the reproducers files I was getting from building CheriBSD
didn't work because the --sysroot option was not being rewritten. I've
updated the test to also verify that the rewritten path matches uses a
FileCheck capature instead of a {{.+}} regex

Reviewers: ruiu

Reviewed By: ruiu

Subscribers: llvm-commits, emaste

Differential Revision: https://reviews.llvm.org/D40125

llvm-svn: 318656