The prefix given to --prefix will be added to GNU absolute paths when
used with --source option (source interleaved with the disassembly).
This matches GNU's objdump behavior.
GNU and C++17 rules for absolute paths are different.
Differential Revision: https://reviews.llvm.org/D85024
Fixes PR46368.
Differential Revision: https://reviews.llvm.org/D85024