opcodes/loongarch: style disassembled address offsets as such

Add a modifier char 'o' telling the disassembler to print the immediate
using the address offset style, and mark the memory access instructions'
offset operands as such.

opcodes/ChangeLog:

	* loongarch-dis.c (dis_one_arg): Style disassembled address
	offsets as such when the operand has a modifier char 'o'.
	* loongarch-opc.c: Add 'o' to operands that represent address
	offsets.

Signed-off-by: WANG Xuerui <git@xen0n.name>