Partly revert 4a8704e

This actually caused more trouble than it fixed. The real reason was
that _POSIX_TIMERS wasn't defined and thus nanosleep() would not be
declared by time.h. libogc however also provides nanosleep(), but an
incompatible one that we don't really want. So after _POSIX_TIMERS has
been defined, it would actually conflict, as both declarations would be
used then. This removes the special handling for Wii and uses the
nanosleep() from time.h, which has a declaration that is compatible with
POSIX.