Revert "windows/mkwinsyscall: use syscall.SyscallN instead of syscall.Syscall{6,9,12,15}"

This reverts CL 614082.

Reason for revert: syscall.SyscallN allocates more than its syscall.SyscallX counterparts, producing perf-related test failures across the board.

Updates #70197

Change-Id: I51107d909fcdbef4e65ee3f84932b2a0e7804f1b
Reviewed-on: https://go-review.googlesource.com/c/sys/+/625375
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>