Fix minor NDS32 renaming snafu

[Jeff Law]

On 7/2/2021 5:21 AM, Alan Modra via Binutils wrote:
> Some extern declarations differ in constness to their definitions too.
> Let's make sure this sort of thing doesn't happen again, but putting
> the externs in a header where they belong.
>
> gas/
> 	* config/tc-nds32.c (nds32_keyword_gpr): Don't declare.
> 	(md_begin): Constify k.
> opcodes/
> 	* nds32-dis.c (nds32_find_reg_keyword): Constify arg and return.
> 	(nds32_parse_audio_ext, nds32_parse_opcode): Constify psys_reg.
> 	(nds32_field_table, nds32_opcode_table, nds32_keyword_table),
> 	(nds32_opcodes, nds32_operand_fields, nds32_keywords),
> 	(nds32_keyword_gpr): Move declarations to..
> 	* nds32-asm.h: ..here, constifying to match definitions.
Thanks.  Both nds targets in my tester started failing yesterday, but I 
hadn't dug into why yet.  I bet you've fixed it.
Jeff