Made osigreturn(2) standard so that SYS_osigreturn can be used in the
signal trampoline for old signals. The arches that support old signals
currently abuse sigreturn(2) instead. This mainly complicates things
and slightly breaks the the new sigreturn(2).

COMPAT is too limited to support the correct configuration of osigreturn,
and this commit doesn't attempt to fix it; it just moves the bogusness:
osigreturn() must now be provided unconditionally even on arches that
don't really need it; previously it had to be provided under the bogus
condition defined(COMPAT_43).