Emulate 64-bit signed high multiplication ((a*b)>>64). To do this
we use the 64-bit unsigned high multiplication method and then
fix the result as shown in Hacker's Delight 2nd ed., chapter 8-3.

Required to enable some division optimizations.

Change-Id: I9194f428e09d3d029cb1afb4715cd5424b5d922e
Reviewed-on: https://go-review.googlesource.com/21774Reviewed-by: Bill O'Farrell <billotosyr@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>