RISC-V: Prepare i18n for required ISA extensions

Some strings returned by the riscv_multi_subset_supports_ext function
contain not just extension names but words like "and" and "or".
This commit wraps such strings with the gettext macro (_) for
internationalization in the future.

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_multi_subset_supports_ext): Wrap some
	strings with the gettext macro to prepare future i18n.