load_aout: replace bswap_needed with big_endian

load_aout: replace bswap_needed with big_endian

Targets know whether they are big-endian more than they know if
the endianness is different from the host: the former is mostly
a constant, at least in machine creation code, while the latter
has to be computed with TARGET_BIG_ENDIAN != HOST_BIG_ENDIAN or
something like that.

load_aout, however, takes a "bswap_needed" argument.  Replace
it with a "big_endian" argument; even though all users are
big-endian, it is cheap enough to keep the optional swapping
functionality even for little-endian boards.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>