Clean up more ancient history: since 2015 the libc stubs for

Clean up more ancient history: since 2015 the libc stubs for
fork/vfork/__tfork haven't cared about the second return register.
So, stop setting retval[1] in kern_fork.c and stop setting the
second return register in the MD child_return() routines.

With the above, we have no multi-register return values on LP64,
so stop touching that register in the trapframe on those archs.

testing miod@ and aoyama@
ok miod@