Compile osigreturn() unconditionally since it will always be needed on

Compile osigreturn() unconditionally since it will always be needed on
some arches and the syscall table is machine-independent.  It was
(bogusly) conditional on COMPAT_43, so this usually makes no difference.

ia64: in addition:
- replace the bogus cloned comment before osigreturn() by a correct one.
  osigreturn() is just a stub fo ia64's.
- fix the formatting of cloned comment before sigreturn().
- fix the return code.  use nosys() instead of returning ENOSYS to get
  the same semantics as if the syscall is not in the syscall table.
  Generating SIGSYS is actually correct here.
- fix style bugs.

powerpc: copy the cleaned up ia64 stub.  This mainly fixes a bogus comment.

sparc64: copy the cleaned up the ia64 stub, since there was no stub before.