Use subsystem to distinguish between pei-arm-little and pei-arm-wince-little

Running objdump against a 32-bit ARM PE file currently needs
disambiguation, as it gets picked up by both pei-arm-little and
pei-arm-wince-little.

This adds a check in pe_bfd_object_p so that the subsystem in the PE
header is used to do the disambiguation for us, so that WinCE images get
assigned to pei-arm-wince-little, and everything else to pei-arm-little.