MIPS16: Reassign `0' and `4' operand codes

MIPS16: Reassign `0' and `4' operand codes

Replace `0' and `4' operand codes with `.' and `F' respectively to free
up the `0'-`4' consecutive range.  No functional change.

	gas/
	* config/tc-mips.c (mips16_macro_build): Replace `0' and `4'
	operand codes with `.' and `F' respectively.
	(mips16_macro): Likewise.

	include/
	* opcode/mips.h: Replace `0' and `4' operand codes with `.' and
	`F' respectively.

	opcodes/
	* mips16-opc.c (decode_mips16_operand): Replace `0' and `4'
	operand codes with `.' and `F' respectively.
	(mips16_opcodes): Likewise.