Fixes #2178.
Patch2: Fixed allocating shadow space for stdcall (must be at least 32 bytes in any case)
Patch3: Made allocated chunk smaller.
Patch4: Typo
Patch5: suppress linktime warning "runtime.callbackasm: nosplit stack overflow"
Patch6: added testcase src/pkg/syscall/callback_windows_test.go
Patch7: weakly related files moved to https://golang.org/cl/4965050 https://golang.org/cl/4974041 https://golang.org/cl/4965051
Patch8: reflect changes https://golang.org/cl/4926042/
Patch9: reflect comments

R=golang-dev, alex.brainman, vcc.163
CC=golang-dev, hectorchu
https://golang.org/cl/4958042