bfd/arc: Rename enum entries to avoid conflicts

In bfd/elf32-arc.c an enum is created that contains entries with generic
names like 'NONE' and 'OFF'.  This has been fine for now, but I had a
need to include opcode/arc.h into bfd/elf32-arc.c.  Unfortunately
opcode/arc.h includes a different enum with identical generic names.

Given that changing the enum in the header file could mean wide-ranging
changes, while changing the enum in the .c file is limited to only
changing the one file, I've added a prefix to the enum in the .c file.

This commit does not add the new include, that will come later.  There
should be no functional change with this commit.

bfd/ChangeLog:

	* elf32-arc.c (tls_got_entries): Add 'TLS_GOT_' prefix to all
	entries.
	(elf_arc_relocate_section): Update enum uses.
	(elf_arc_check_relocs): Likewise.
	(elf_arc_finish_dynamic_symbol): Likewise.