windows/386: clean stack after syscall (it is necessary after call cdecl functions and does not have an effect after stdcall)

Result of discussion here: http://groups.google.com/group/golang-nuts/browse_thread/thread/357c806cbb57ca62

R=golang-dev, bradfitz, alex.brainman, hectorchu, rsc
CC=golang-dev
https://golang.org/cl/4961045