eina: handle more compiler strangeness for bswap.

eina: handle more compiler strangeness for bswap.

So current order is :
- __builtin_bswap*() for compiler that provide it
- _byteswap_*() for MSVC
- bswap_*() for older Linux and some BSD
- own C code when everything else fall appart.

The reason for this order is that the builtin will always generate
the best assembly possible. On my system bswap_*() are not changing
in all version to the best solution as they are almost equivalent to
the C macro.