windows/386 also wants underscore prefix for external names.
This CL is in preparation of external linking support.

Change-Id: I2d2ea233f976aab3f356f9b508cdd246d5013e2d
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/7282Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>