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

Two code paths may reach the EHFrame case in SectionBase::getOffset:

* .eh_frame reference
* relocation copy for --emit-relocs

The first may be used by clang_rt.crtbegin.o and GCC crtbeginT.o to get the
start address of the output .eh_frame. The relocation has an offset of 0 or
(x86-64 PC-relative leaq for clang_rt.crtbegin.o) -4. The current code just
returns `offset`, which handles this case well.

The second is related to InputSection::copyRelocations on .eh_frame (used by
--emit-relocs). .eh_frame pieces may be dropped due to GC/ICF, so we should
convert the input offset to the output offset. 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.

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