runtime: library entry point for darwin/arm

Tested by using -buildmode=c-archive to generate an archive, add it
to an Xcode project and calling a Go function from an iOS app. (I'm
still investigating proper buildmode tests for all.bash.)

Change-Id: I7890df15246df8e90ad27837b8d64ba2cde409fe
Reviewed-on: https://go-review.googlesource.com/8719Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/rt0_darwin_arm.s

@@ -11,6 +11,47 @@ TEXT _rt0_arm_darwin(SB),7,$-4
 	MOVW	$main(SB), R4
 	B		(R4)
 
+// When linking with -buildmode=c-archive or -buildmode=c-shared,
+// this symbol is called from a global initialization function.
+//
+// Note that all currently shipping darwin/arm platforms require
+// cgo and do not support c-shared.
+TEXT _rt0_arm_darwin_lib(SB),NOSPLIT,$12
+	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
+	B.EQ  nocgo
+	MOVW  $_rt0_arm_darwin_lib_go(SB), R0
+	MOVW  $0, R1
+	BL    (R4)
+	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)
+	RET
+
+TEXT _rt0_arm_darwin_lib_go(SB),NOSPLIT,$0
+	MOVW  _rt0_arm_darwin_lib_argc<>(SB), R0
+	MOVW  _rt0_arm_darwin_lib_argv<>(SB), R1
+	MOVW  R0,  (R13)
+	MOVW  R1, 4(R13)
+	MOVW  $runtime·rt0_go(SB), R4
+	B     (R4)
+
+DATA  _rt0_arm_darwin_lib_argc<>(SB)/4, $0
+GLOBL _rt0_arm_darwin_lib_argc<>(SB),NOPTR, $4
+DATA  _rt0_arm_darwin_lib_argv<>(SB)/4, $0
+GLOBL _rt0_arm_darwin_lib_argv<>(SB),NOPTR, $4
+
 TEXT main(SB),NOSPLIT,$-8
 	// save argc and argv onto stack
 	MOVM.DB.W [R0-R1], (R13)