misc/cgo/test: fix issue9400 test on android/386

The test for #9400 relies on an assembler function that manipulates
the stack pointer. Meanwile, it uses a global variable for
synchronization. However, position independent code on 386 use a
function call to fetch the base address for global variables.
That function call in turn overwrites the Go stack.

Fix that by fetching the global variable address once before the
stack register manipulation.

Fixes the android/386 builder.

Change-Id: Ib77bd80affaa12f09d582d09d8b84a73bd021b60
Reviewed-on: https://go-review.googlesource.com/23683
Run-TryBot: Elias Naur <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: David Crawshaw <[email protected]>

Author: Elias Naur

misc/cgo/test/issue9400/asm_386.s

@@ -7,17 +7,18 @@
 #include "textflag.h"
 
 TEXT ·RewindAndSetgid(SB),NOSPLIT,$0-0
+	MOVL	$·Baton(SB), BX
 	// Rewind stack pointer so anything that happens on the stack
 	// will clobber the test pattern created by the caller
 	ADDL	$(1024 * 8), SP
 
 	// Ask signaller to setgid
-	MOVL	$1, ·Baton(SB)
+	MOVL	$1, (BX)
 
 	// Wait for setgid completion
 loop:
 	PAUSE
-	MOVL	·Baton(SB), AX
+	MOVL	(BX), AX
 	CMPL	AX, $0
 	JNE	loop