RISC-V: Shrink the riscv_implicit_subsets table.

Allow to add implicit extensions by using the syntax of `.option arch, +-', so
that the table is shrinked and more readable.

bfd/
	* elfxx-riscv.c (check_implicit_always): Removed the unused IMPLICIT
	parameter.
	(check_implicit_for_i): Likewise.
	(riscv_implicit_subsets): Shrink the table by allowing the syntax of
	`.option arch, +-' for implicit extensions.
	(riscv_update_subset1): New function, called from riscv_update_subset
	or riscv_parse_add_implicit_subsets.  It basically does the same thing
	as riscv_update_subset function before.
	(riscv_parse_add_implicit_subsets): Updated.
	(riscv_update_subset): Updated.