strconv: fix performance regression in integer formatting on 32bit platforms

strconv: fix performance regression in integer formatting on 32bit platforms

Some of the changes in CL golang.org/cl/38071/ assumed that / and %
could always be combined to use only one DIV instruction. However,
this is not the case for 64bit operands on a 32bit platform which use
seperate runtime functions to calculate division and modulo.

This CL restores the original optimizations that help on 32bit platforms
with negligible impact on 64bit platforms.

386:
name          old time/op  new time/op  delta
FormatInt-2   6.06µs ± 0%  6.02µs ± 0%  -0.70%  (p=0.000 n=20+20)
AppendInt-2   4.98µs ± 0%  4.98µs ± 0%    ~     (p=0.747 n=18+18)
FormatUint-2  1.93µs ± 0%  1.85µs ± 0%  -4.19%  (p=0.000 n=20+20)
AppendUint-2  1.71µs ± 0%  1.64µs ± 0%  -3.68%  (p=0.000 n=20+20)

amd64:
name          old time/op  new time/op  delta
FormatInt-2   2.41µs ± 0%  2.41µs ± 0%  -0.09%  (p=0.010 n=18+18)
AppendInt-2   1.77µs ± 0%  1.77µs ± 0%  +0.08%  (p=0.000 n=18+18)
FormatUint-2   653ns ± 1%   653ns ± 0%    ~     (p=0.178 n=20+20)
AppendUint-2   514ns ± 0%   513ns ± 0%  -0.13%  (p=0.000 n=20+17)

Change-Id: I574a18e54fb41b25fbe51ce696e7a8765abc79a6
Reviewed-on: https://go-review.googlesource.com/38051
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>