[GOLD] -Wimplicit-fallthrough warning fixes

[GOLD] -Wimplicit-fallthrough warning fixes

	* aarch64.cc: Spell fall through comments as "// Fall through.".
	* arm.cc: Likewise.
	* mips.cc: Likewise.
	* powerpc.cc: Likewise.
	* s390.cc: Likewise.
	* sparc.cc: Likewise.
	* x86_64.cc: Likewise.
	* powerpc.cc (Target_powerpc::Relocate::relocate): Add missing
	fall through comments.
	* sparc.cc: (Target_sparc::Scan::global): Likewise.
	(Target_sparc::Relocate::relocate): Likewise.
	* tilegx.cc (Target_tilegx::Relocate::relocate): Likewise.
	* resolve.cc (symbol_to_bits): Add missing break.