cmd/6g: allow use of R14, R15 now
cmd/6g: allow use of R14, R15 now

We stopped reserving them in 2009 or so.

R=ken
CC=golang-dev
http://codereview.appspot.com/6215061

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang