cmd/cc: add PREFETCH built-in (like SET, USED)

cmd/cc: add PREFETCH built-in (like SET, USED)

This makes it possible to inline the prefetch of upcoming
memory addresses during garbage collection, instead of
needing to flush registers, make a function call, and
reload registers.  On garbage collection-heavy workloads,
this results in a 5% speedup.

Fixes #3493.

R=dvyukov, ken, r, dave
CC=golang-dev
https://golang.org/cl/5990066