riscv: print .2byte or .4byte before an unknown instruction encoding

riscv: print .2byte or .4byte before an unknown instruction encoding

When the RISC-V disassembler encounters an unknown instruction, it
currently just prints the value of the bytes, like this:

  Dump of assembler code for function custom_insn:
     0x00010132 <+0>:	addi	sp,sp,-16
     0x00010134 <+2>:	sw	s0,12(sp)
     0x00010136 <+4>:	addi	s0,sp,16
     0x00010138 <+6>:	0x52018b
     0x0001013c <+10>:	0x9c45

My proposal, in this patch, is to change the behaviour to this:

  Dump of assembler code for function custom_insn:
     0x00010132 <+0>:	addi	sp,sp,-16
     0x00010134 <+2>:	sw	s0,12(sp)
     0x00010136 <+4>:	addi	s0,sp,16
     0x00010138 <+6>:	.4byte	0x52018b
     0x0001013c <+10>:	.2byte	0x9c45

Adding the .4byte and .2byte opcodes.  The benefit that I see here is
that in the patched version of the tools, the disassembler output can
be fed back into the assembler and it should assemble to the same
binary format.  Before the patch, the disassembler output is invalid
assembly.

I've started a RISC-V specific test file under binutils so that I can
add a test for this change.

binutils/ChangeLog:

	* testsuite/binutils-all/riscv/riscv.exp: New file.
	* testsuite/binutils-all/riscv/unknown.d: New file.
	* testsuite/binutils-all/riscv/unknown.s: New file.

opcodes/ChangeLog:

	* riscv-dis.c (riscv_disassemble_insn): Print a .%dbyte opcode
	before an unknown instruction, '%d' is replaced with the
	instruction length.