runtime: shared library init support for android/arm.

Follows http://golang.org/cl/8454, a similar CL for arm architectures.
This CL involves android-specific changes, namely, synthesizing
argv/auxv, as android doesn't provide those to the init functions.

This code is based on crawshaw@ android code in golang.org/x/mobile.

Change-Id: I32364efbb2662e80270a99bd7dfb1d0421b5417d
Reviewed-on: https://go-review.googlesource.com/8457Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/runtime/rt0_android_arm.s

@@ -10,8 +10,26 @@ TEXT _rt0_arm_android(SB),NOSPLIT,$-4
 	MOVW		$_rt0_arm_linux1(SB), R4
 	B		(R4)
 
-// This symbol is called when a shared library is loaded.
+// When building with -buildmode=c-shared, this symbol is called when the shared
+// library is loaded.
 TEXT _rt0_arm_android_lib(SB),NOSPLIT,$0
-	// TODO(crawshaw): initialize runtime.
-	// At the moment this is done in mobile/app/android.c:init_go_runtime
+	MOVW	$1, R0                          // argc
+	MOVW	$_rt0_arm_android_argv(SB), R1  // **argv
+	BL _rt0_arm_linux_lib(SB)
 	RET
+
+DATA _rt0_arm_android_argv+0x00(SB)/4,$_rt0_arm_android_argv0(SB)
+DATA _rt0_arm_android_argv+0x04(SB)/4,$0
+DATA _rt0_arm_android_argv+0x08(SB)/4,$0
+DATA _rt0_arm_android_argv+0x0C(SB)/4,$15      // AT_PLATFORM
+DATA _rt0_arm_android_argv+0x10(SB)/4,$_rt0_arm_android_auxv0(SB)
+DATA _rt0_arm_android_argv+0x14(SB)/4,$16      // AT_HWCAP
+DATA _rt0_arm_android_argv+0x18(SB)/4,$0x2040  // HWCAP_VFP | HWCAP_VFPv3
+DATA _rt0_arm_android_argv+0x1C(SB)/4,$0
+GLOBL _rt0_arm_android_argv(SB),NOPTR,$0x20
+
+DATA _rt0_arm_android_argv0(SB)/8, $"gojni"
+GLOBL _rt0_arm_android_argv0(SB),RODATA,$8
+
+DATA _rt0_arm_android_auxv0(SB)/4, $"v7l"
+GLOBL _rt0_arm_android_auxv0(SB),RODATA,$4

src/runtime/rt0_linux_amd64.s

@@ -10,8 +10,8 @@ TEXT _rt0_amd64_linux(SB),NOSPLIT,$-8
 	MOVQ	$main(SB), AX
 	JMP	AX
 
-// When linking with -shared, this symbol is called when the shared library
-// is loaded.
+// When building with -buildmode=c-shared, this symbol is called when the shared
+// library is loaded.
 TEXT _rt0_amd64_linux_lib(SB),NOSPLIT,$40
 	MOVQ	DI, _rt0_amd64_linux_lib_argc<>(SB)
 	MOVQ	SI, _rt0_amd64_linux_lib_argv<>(SB)

src/runtime/rt0_linux_arm.s

@@ -10,6 +10,8 @@ TEXT _rt0_arm_linux(SB),NOSPLIT,$-4
 	MOVW	$_rt0_arm_linux1(SB), R4
 	B		(R4)
 
+// When building with -buildmode=c-shared, this symbol is called when the shared
+// library is loaded.
 TEXT _rt0_arm_linux_lib(SB),NOSPLIT,$40
 	// Preserve callee-save registers.  Raspberry Pi's dlopen(), for example,
 	// actually cares that R11 is preserved.