Don't generate relocations for removed symbols.

A symbol within a garbage collected section is still trying to have a
relocation reference it.  Don't do that, just generate an R_ARC_NONE
relocation instead.

bfd/ChangeLog:

	* elf32-arc.c (elf_arc_finish_dynamic_symbol): Don't generate
	R_ARC_GLOB_DAT relocations for symbols that have been removed.
Fix relocation weak symbols
Handle prefetchw, prefetch.l2
Fix mbarrel-shifter option for ARCv2