Second argument of servicemain is a pointer. See

https://msdn.microsoft.com/en-us/library/windows/desktop/ms685138(v=vs.85).aspx

So amd64 assembler code should use MOVQ to read that value.
I probably copied 386 assembler code into amd64 and did not
adjust the code.

Broken code was used to pass parameters to the service, so
add some tests to verify that parameter passing works.

Fixes golang/go#24575

Change-Id: I89f8cad026ea13f8a5d78ff3e24b7236e27fc91f
Reviewed-on: https://go-review.googlesource.com/110160
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>