syscall: fix ComputerName on Windows

syscall: fix ComputerName on Windows

GetComputerName expects n to be the size of the buffer, and
on output contains the number of characters copied to the buffer.

CL 493036 broke ComputerName by always setting n to 0.

Change-Id: I3f4b30d2f9825d321a6d28ec82bdc7b6294e04e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/499035
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>