Fix bug with relocation addends and merge sections with --icf.

Fix bug with relocation addends and merge sections with --icf.

During --icf processing, gold was incorrectly processing the relocation
addend for references to items in a merge section. PC-relative references
and other forms of reference with a biased base address require a
non-section local symbol, where the addend is purely the bias.

gold/
	PR gold/20642
	PR gold/22820
	* gc.h (gc_process_relocs): Flag STT_SECTION symbols in symvec.
	* icf.cc (get_section_contents): For merge sections, ignore the
	addend for relocations against non-section symbols.