[release-branch.go1.5] runtime: preserve R11 in darwin/arm entrypoint

The _rt0_arm_darwin_lib entrypoint has to conform to the darwin ARMv7
calling convention, which requires functions to preserve the value of
R11. Go uses R11 as the liblink REGTMP register, so save it manually.

Also avoid using R4, which is also callee-save.

Fixes #12590

Change-Id: I9c3b374e330f81ff8fc9c01fa20505a33ddcf39a
Reviewed-on: https://go-review.googlesource.com/14603
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-on: https://go-review.googlesource.com/16968
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/rt0_darwin_arm.s

@@ -16,27 +16,34 @@ TEXT _rt0_arm_darwin(SB),7,$-4
 //
 // Note that all currently shipping darwin/arm platforms require
 // cgo and do not support c-shared.
-TEXT _rt0_arm_darwin_lib(SB),NOSPLIT,$12
+TEXT _rt0_arm_darwin_lib(SB),NOSPLIT,$0
+	// R11 is REGTMP, reserved for liblink. It is used below to
+	// move R0/R1 into globals. However in the darwin ARMv7 calling
+	// convention, it is a callee-saved register. So we save it to a
+	// temporary register.
+	MOVW  R11, R2
 	MOVW  R0, _rt0_arm_darwin_lib_argc<>(SB)
 	MOVW  R1, _rt0_arm_darwin_lib_argv<>(SB)
 
 	// Create a new thread to do the runtime initialization and return.
-	MOVW  _cgo_sys_thread_create(SB), R4
-	CMP   $0, R4
+	MOVW  _cgo_sys_thread_create(SB), R3
+	CMP   $0, R3
 	B.EQ  nocgo
 	MOVW  $_rt0_arm_darwin_lib_go(SB), R0
 	MOVW  $0, R1
-	BL    (R4)
+	MOVW  R2, R11
+	BL    (R3)
 	RET
 nocgo:
 	MOVW  $0x400000, R0
-	MOVW  $_rt0_arm_darwin_lib_go(SB), R1
-	MOVW  $0, R2
-	MOVW  R0,  (R13) // stacksize
-	MOVW  R1, 4(R13) // fn
-	MOVW  R2, 8(R13) // fnarg
-	MOVW  $runtime·newosproc0(SB), R4
-	BL    (R4)
+	MOVW  R0, (R13) // stacksize
+	MOVW  $_rt0_arm_darwin_lib_go(SB), R0
+	MOVW  R0, 4(R13) // fn
+	MOVW  $0, R0
+	MOVW  R0, 8(R13) // fnarg
+	MOVW  $runtime·newosproc0(SB), R3
+	MOVW  R2, R11
+	BL    (R3)
 	RET
 
 TEXT _rt0_arm_darwin_lib_go(SB),NOSPLIT,$0