[LLD][ELF] Eliminate symbols of merged .ARM.exidx sections.

GNU tools generate mapping symbols "$d" for .ARM.exidx sections. The
symbols are added to the symbol table much earlier than the merging
takes place, and after that, they become dangling. Before the patch,
LLD output those symbols as SHN_ABS with the value of 0. The patch
removes such symbols from the symbol table.

Differential Revision: https://reviews.llvm.org/D78820