[dev.garbage] runtime: add prefetcht0, prefetcht1, prefetcht2, prefetcht3, prefetchnta for GC

We don't know what we need yet, so add them all.
Add them even on x86 architectures (as no-ops) so that
the GC can refer to them unconditionally.

Eventually we'll know what we want and probably
have just one 'prefetch' with an appropriate meaning
on each architecture.

LGTM=rlh
R=rlh
CC=golang-codereviews
https://codereview.appspot.com/179160043