runtime: mark endcgo go:nosplit

Should help windows/amd64.

TBR=khr
CC=golang-codereviews
https://codereview.appspot.com/134660043
runtime: turn off StackCopyAlways

windows/amd64 failure:
http://build.golang.org/log/1ded5e3ef4bd1226f976e3180772f87e6c918255

# ..\misc\cgo\testso
runtime: copystack: locals size info only for syscall.Syscall
fatal error: split stack not allowed

runtime stack:
runtime.throw(0xa64cc7)
        c:/go/src/runtime/panic.go:395 +0xad fp=0x6fde0 sp=0x6fdb0
runtime.newstack()
        c:/go/src/runtime/stack.c:1001 +0x750 fp=0x6ff20 sp=0x6fde0
runtime.morestack()
        c:/go/src/runtime/asm_amd64.s:306 +0x73 fp=0x6ff28 sp=0x6ff20

goroutine 1 [stack growth, locked to thread]:
runtime.freedefer(0xc0820ce120)
        c:/go/src/runtime/panic.go:162 fp=0xc08201b1a0 sp=0xc08201b198
runtime.deferreturn(0xa69420)
        c:/go/src/runtime/panic.go:211 +0xa8 fp=0xc08201b1e8 sp=0xc08201b1a0
runtime.cgocall_errno(0x498c00, 0xc08201b228, 0x0)
        c:/go/src/runtime/cgocall.go:134 +0x10e fp=0xc08201b210 sp=0xc08201b1e8
syscall.Syscall(0x7786b1d0, 0x2, 0xc0820c85b0, 0xc08201b2d8, 0x0, 0x0, 0x0, 0x0)
        c:/go/src/runtime/syscall_windows.c:74 +0x3c fp=0xc08201b260 sp=0xc08201b210
syscall.findFirstFile1(0xc0820c85b0, 0xc08201b2d8, 0x500000000000000, 0x0, 0x0)
        c:/go/src/syscall/zsyscall_windows.go:340 +0x76 fp=0xc08201b2b0 sp=0xc08201b260
syscall.FindFirstFile(0xc0820c85b0, 0xc08210c500, 0xc0820c85b0, 0x0, 0x0)
        c:/go/src/syscall/syscall_windows.go:907 +0x6a fp=0xc08201b530 sp=0xc08201b2b0
os.openDir(0xc0820b2e40, 0x33, 0x0, 0x0, 0x0)
        c:/go/src/os/file_windows.go:96 +0x110 fp=0xc08201b5e0 sp=0xc08201b530
os.OpenFile(0xc0820b2e40, 0x33, 0x0, 0x0, 0x41, 0x0, 0x0)
        c:/go/src/os/file_windows.go:143 +0x1e9 fp=0xc08201b650 sp=0xc08201b5e0

TBR=khr
CC=golang-codereviews
https://codereview.appspot.com/138230043