BUG: Fix byteswapping for complex scalars

During a cleanup, the fast paths were invalidated because SIZEOF_LONGDOUBLE
was not defined anymore and needs to be replaced with NPY_SIZEOF_LONGDOUBLE.
The other SIZEOF macros still existed however so only complex long double
broke because it switched to the already broken fast path.

This commit fixes the fast path, and replaces all SIZEOF_ macros within
arraytypes.c.src with their corresponding NPY_SIZEOF_ macros.
Merge pull request #2886 from seberg/complex256-byteswap

BUG: Fix byteswapping for complex scalars