syscall: replace mksyscall_windows.pl with mksyscall_windows.go

Not many windows users have perl installed. They can just use
standard go tools instead. Also mkerrors_windows.sh script
removed - we don't add any new "unix" errors to windows
syscall package anymore.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/41060044
7 files changed