go.sys/windows: grab Syscall interface from the standard syscall package
Also provide stubs to call loadlibrary and getprocaddress, which are implemented
for package syscall by runtime/syscall_windows.goc.
Should get the windows go.sys build closer to green.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/126170043
6 files changed