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.
BUG: Replace unprefixed SIZEOF_* macros with prefixed versions.

The sources don't define NPY_NO_PREFIX and consequently none of the
unprefixed macros are defined. Using them can lead to some unexpected
results.
Merge pull request #2900 from certik/backport2886and7

Backport2886and7