RISC-V: Improve "bits undefined" diagnostics

This commit improves internal error message
"internal: bad RISC-V opcode (bits 0x%lx undefined): %s %s"
to display actual unused bits (excluding non-instruction bits).

gas/ChangeLog:

	* config/tc-riscv.c (validate_riscv_insn): Exclude non-
	instruction bits from displaying internal diagnostics.
	Change error message slightly.