runtime: save 8 stack bytes in timediv on arm.
runtime: save 8 stack bytes in timediv on arm.

Operations on int64 are very stack consuming with 5c.
Fixes netbsd/arm build.

Before: TEXT    runtime.timediv+0(SB),7,$52-16
After:  TEXT    runtime.timediv+0(SB),7,$44-16

The stack usage is unchanged on 386:
        TEXT    runtime.timediv+0(SB),7,$8-16

R=golang-dev, dvyukov, bradfitz
CC=golang-dev
https://codereview.appspot.com/12182044

mhennings/marcohennings-go,bryanxu/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,mhennings/marcohennings-go,d0f/go-zh,rdp/rogerpack2005-golang,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,webfd/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,d0f/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,webfd/go-zh,mhennings/marcohennings-go,mhennings/marcohennings-go,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh