[ELF] Use stable_sort for compareByFilePosition
[ELF] Use stable_sort for compareByFilePosition

The comment at the top of compareByFilePosition indicates that it relies
on stable_sort to preserve the order of synthetic sections. We were
using sort instead of stable_sort, however, leading to incorrect
synthetic section ordering.

Differential Revision: https://reviews.llvm.org/D35473

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@308207 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld