native: workaround for missing __builtin_bswap16

Implements a workaround for
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52624

This is present in current versions of ubuntu 12.04 and debian 7.7 (stable).
Merge pull request #2170 from LudwigOrtmann/native-bswap

native: workaround for missing __builtin_bswap16