[ELF] --emit-relocs: adjust offsets of .rel[a].eh_frame relocations

.eh_frame pieces may be dropped due to GC/ICF. When --emit-relocs adds
relocations against .eh_frame, the offsets need to be adjusted. Use the same
way as MergeInputSection with a special case handling outSecOff==-1 for an
invalid piece (see eh-frame-marker.s).

This exposes an issue in mips64-eh-abs-reloc.s that we don't reliably
handle anyway. Just add --no-check-dynamic-relocations to paper over it.

Original patch by Ayrton Muñoz

Differential Revision: https://reviews.llvm.org/D122459