syscall: rewrite Windows makeCmdLine to use []byte

It's faster to append to a []byte and only convert to string at the
end then it is to build up a string by concatenating characters.

Fixes #41825

Change-Id: I45ddf77dcc62726c919f0533c95d483cee8ba366
Reviewed-on: https://go-review.googlesource.com/c/go/+/259978


Trust: Ian Lance Taylor <iant@golang.org>
Trust: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>