objdump cant correctly dissemble instructions that reuse other's encoding

[linsinan1995]

for example, lwgp in zceb extension(zceb do not co-exist with D extension) reuses the encoding of fld, but

lwgp x0,4

will be dissembled into

00403007   fld ft0,4(zero)

it is also a problem for c.decbnez instructions in zceb extenstion.

maybe we can add an arch option to the disassembler, and the disassembler can skip those unsupported instructions based on the given arch string?