hw/core/generic-loader: Compile only once, not for each target

hw/core/generic-loader: Compile only once, not for each target

The generic-loader is currently compiled target specific due to one
single "#ifdef TARGET_WORDS_BIGENDIAN" in the file. We have already a
function called target_words_bigendian() for this instead, so we can
put the generic-loader into common-obj to save some compilation time.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>