Posix core files sometime don't contain enough information to correctly
detect the OS. If that is the case we should use the OS from the target
instead as it will contain usable information in more cases and if the
target and the core contain different OS-es then we are already in a
pretty bad state so moving from an unknown OS to a known (but possibly
incorrect) OS will do no harm.
We already had similar code in place for MIPS. This change tries to make
it more generic by using ArchSpec::MergeFrom and extends it to all
architectures but some MIPS specific issue prevent us from getting rid
of special casing MIPS.