gdb/riscv: Fix some ARI issues

Fix some ARI issues in recently added riscv code, the ARI email is:

  https://sourceware.org/ml/gdb-patches/2018-03/msg00156.html

gdb/ChangeLog:

	* riscv-tdep.c (riscv_sw_breakpoint_from_kind): Add localization
	to error message string.
	(riscv_register_name): Use xsnprintf instead of sprintf.
	(riscv_insn::fetch_instruction): Use gdb_assert instead of
	internal_error.
	(riscv_print_arg_location): Use gdb_assert_not_reached instead of
	error.
	(riscv_push_dummy_call): Likewise.