cgo: windows/386 port

R=rsc, peterGo, brainman
CC=golang-dev
https://golang.org/cl/3733046

src/pkg/runtime/386/asm.s

diff --git a/src/pkg/runtime/386/asm.s b/src/pkg/runtime/386/asm.s
index 101a0cf..58ca712 100644
--- a/src/pkg/runtime/386/asm.s
+++ b/src/pkg/runtime/386/asm.s
@@ -18,9 +18,10 @@
 	// we set up GS ourselves.
 	MOVL	initcgo(SB), AX
 	TESTL	AX, AX
-	JZ	3(PC)
+	JZ	4(PC)
 	CALL	AX
-	JMP	ok
+	CMPL runtime·iswindows(SB), $0
+	JEQ ok
 
 	// set up %gs
 	CALL	runtime·ldt0setup(SB)