runtime: save r11 in ARM addmoduledata

R11 is callee-save in the C ABI, but the temporary register in the Go
ABI. Currently it's being clobbered by runtime.addmoduledata, which
has to follow the C ABI. The observed effect of this was that
dl_open_worker was returning to a bad PC because after it failed to
restore its SP because it was using R11 as a frame pointer.

Fix this by saving R11 around addmoduledata.

Fixes #19674.

Change-Id: Iaacbcc76809a3aa536e9897770831dcbcb6c8245
Reviewed-on: https://go-review.googlesource.com/47831
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/runtime/asm_arm.s

@@ -1011,11 +1011,13 @@ TEXT runtime·sigreturn(SB),NOSPLIT,$0-0
 
 #ifndef GOOS_nacl
 // This is called from .init_array and follows the platform, not Go, ABI.
-TEXT runtime·addmoduledata(SB),NOSPLIT,$0-4
+TEXT runtime·addmoduledata(SB),NOSPLIT,$0-8
 	MOVW	R9, saver9-4(SP) // The access to global variables below implicitly uses R9, which is callee-save
+	MOVW	R11, saver11-8(SP) // Likewise, R11 is the temp register, but callee-save in C ABI
 	MOVW	runtime·lastmoduledatap(SB), R1
 	MOVW	R0, moduledata_next(R1)
 	MOVW	R0, runtime·lastmoduledatap(SB)
+	MOVW	saver11-8(SP), R11
 	MOVW	saver9-4(SP), R9
 	RET
 #endif