misc/cgo/test/issue9400: add arm64 implementation

Change-Id: I309e3df7608b9eef9339196fdc50dedf5f9439f4
Reviewed-on: https://go-review.googlesource.com/8451Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>

misc/cgo/test/issue9400/asm_arm64.s

+#include "textflag.h"
+
+TEXT ·RewindAndSetgid(SB),NOSPLIT,$-8-0
+	// Save link register
+	MOVD	R30, R9
+
+	// Rewind stack pointer so anything that happens on the stack
+	// will clobber the test pattern created by the caller
+	ADD	$(1024 * 8), RSP
+
+	// Ask signaller to setgid
+	MOVD	$·Baton(SB), R0
+	MOVD	$1, R1
+storeloop:
+	LDAXRW	(R0), R2
+	STLXRW	R1, (R0), R3
+	CBNZ	R3, storeloop
+
+	// Wait for setgid completion
+	MOVW	$0, R1
+	MOVW	$0, R2
+loop:
+	LDAXRW	(R0), R3
+	CMPW	R1, R3
+	BNE	loop
+	STLXRW	R2, (R0), R3
+	CBNZ	R3, loop
+
+	// Restore stack
+	SUB	$(1024 * 8), RSP
+
+	MOVD	R9, R30
+	RET