libplist FTBFS on armhf with bus errors

[SRU Justification]
libplist currently fails to build on armhf with bus errors due to unaligned access, making this not currently security supportable as of 18.04 release. Since this library is seeded on all desktop images I am not looking to have this included during the freeze but am making it an SRU instead.

[Test case]
If the package builds from source on all architectures, it has passed its test suite showing that the new code works on all architectures and produces correct results.

[Regression potential]
It is possible there may be performance regressions on some other little-endian architectures besides armhf as a result of this change to the byte swap implementation. I do not think this would be the case. I don't know if this code path is performance-sensitive.