syscall: another attempt to keep windows syscall pointers live

This approach was suggested in
https://codereview.appspot.com/138250043/#msg15.
Unlike current version of mksyscall_windows.go,
new code could be used in go.sys and other external
repos without help from asm.

LGTM=iant
R=golang-codereviews, iant, r
CC=golang-codereviews
https://codereview.appspot.com/143160046