runtime: don't use thread local storage before it is set up on iOS

CL 138675 added a call to runtime.save_g which uses thread local
storage to store g. On iOS however, that storage was not initialized
 yet. Move the call to below _cgo_init where it is set up.

Change-Id: I14538d3e7d56ff35a6fa02c47bca306d24c38010
Reviewed-on: https://go-review.googlesource.com/c/150157
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/runtime/asm_arm64.s

@@ -18,7 +18,6 @@ TEXT runtime·rt0_go(SB),NOSPLIT,$0
 	// create istack out of the given (operating system) stack.
 	// _cgo_init may update stackguard.
 	MOVD	$runtime·g0(SB), g
-	BL	runtime·save_g(SB)
 	MOVD	RSP, R7
 	MOVD	$(-64*1024)(R7), R0
 	MOVD	R0, g_stackguard0(g)
@@ -45,6 +44,7 @@ TEXT runtime·rt0_go(SB),NOSPLIT,$0
 	ADD	$16, RSP
 
 nocgo:
+	BL	runtime·save_g(SB)
 	// update stackguard after _cgo_init
 	MOVD	(g_stack+stack_lo)(g), R0
 	ADD	$const__StackGuard, R0