MIPS/opcodes: Disassemble the RFE instruction

MIPS/opcodes: Disassemble the RFE instruction

Fix a commit b015e599c772 ("[MIPS] Add new virtualization instructions"),
<https://sourceware.org/ml/binutils/2013-05/msg00118.html>, regression
and bring the disassembly of the RFE instruction back for the relevant
ISA levels.

It is because the "rfe" opcode table entry was incorrectly moved behind
the catch-all generic "c0" entry for CP0 instructions, causing output
like:

  00:	42000010 	c0	0x10

to be produced rather than:

  00:	42000010 	rfe

even for ISA levels that do include the RFE instruction.

Move the "rfe" entry ahead of "c0" then, correcting the problem.  Add a
suitable test case.

	opcodes/
	* mips-opc.c (mips_builtin_opcodes): Move the "rfe" entry ahead
	of "c0".

	gas/
	* testsuite/gas/mips/rfe.d: New test.
	* testsuite/gas/mips/rfe.s: New test source.
	* testsuite/gas/mips/mips.exp: Run the new test.