[PATCH 16/19] RISC-V: improve a subset parsing diagnostic

[Jiawei]

> These sets can be long, so report the specific item along with the full
> argument.
>
> --- a/bfd/elfxx-riscv.c
> +++ b/bfd/elfxx-riscv.c
> @@ -2688,8 +2688,8 @@ riscv_update_subset1 (riscv_parse_subset
>      {
>        q[1] = '\0';
>        rps->error_handler
> -        (_("%sinvalid ISA extension ends with <number>p in %s `%s'"),
> -           errmsg_internal, errmsg_caller, implicit_exts);
> +        (_("%sinvalid ISA extension `%s' ends with <number>p in %s 
> `%s'"),
> +           errmsg_internal, subset, errmsg_caller, implicit_exts);
>        free (subset);
>        return false;
>      }
> --- a/gas/testsuite/gas/riscv/option-arch-fail.l
> +++ b/gas/testsuite/gas/riscv/option-arch-fail.l
> @@ -7,5 +7,5 @@
>  .*Error: unknown ISA extension `zsubset' in .option arch `\+zsubset2p0'
>  .*Error: unknown ISA extension `f2p0_d' in .option arch `\+f2p0_d2p0'
>  .*Error: unknown ISA extension `' in .option arch `\+'
> -.*Error: invalid ISA extension ends with <number>p in .option arch 
> `\+xvendor2p'
> +.*Error: invalid ISA extension `xvendor2p' ends with <number>p in 
> .option arch `\+xvendor2p'
>  .*Error: .option pop with no .option push
>
LGTM.

Including the specific invalid subset makes this diagnostic easier to
understand when the full subset argument is long.

Reviewed-by: Jiawei <jiawei@iscas.ac.cn>