- use proper Win64 gcc calling convention when
  calling initcgo on amd64
- increase g0 stack size to 64K on amd64 to make
  it the same as 386
- implement C.sleep
- do not use C.stat, since it is renamed to C._stat by mingw
- use fopen to implement TestErrno, since C.strtol
  always succeeds on windows
- skip TestSetEnv on windows, because os.Setenv
  sets windows process environment, while C.getenv
  inspects internal C runtime variable instead

R=golang-dev, vcc.163, rsc
CC=golang-dev
https://golang.org/cl/5500094