Fix segv when unwinding stack past __libc_init.

This change fixes a segmentation fault in the libc unwinder when it goes
past __libc_init.

Unwind instructions for __libc_init direct it to grab the return address from
the stack frame. Without this change, the unwinder gets a wild address and
looks up further unwind instructions for the routine at that address. If it's
unlucky enough to hit an existing function, it will try to unwind it. Bad
things happen then.

With this change, the return address always points to the _start function,
which does not have unwind instructions associated with it. This stop the
unwind process.

__libc_init never returns, so this does not affect program execution, other
than adding 4 bytes on the main thread stack.

Change-Id: Id58612172e8825c8729cccd081541a13bff96bd0
diff --git a/libc/arch-arm/bionic/crtbegin_dynamic.S b/libc/arch-arm/bionic/crtbegin_dynamic.S
index 6ca0845..ec6d482 100644
--- a/libc/arch-arm/bionic/crtbegin_dynamic.S
+++ b/libc/arch-arm/bionic/crtbegin_dynamic.S
@@ -53,7 +53,9 @@
 	ldr	r2, =main
 	adr	r3, 1f
 	ldr	r4, =__libc_init
-	bx	r4
+	blx	r4
+	mov	r0, #0
+	bx	r0
 1:  .long   __PREINIT_ARRAY__
     .long   __INIT_ARRAY__