Clear the return address register in __bionic_clone.

The unwinder will end up on the parent thread's stack if the return
address register is not cleared.

Bug: 14270816
Bug: 15195760
Change-Id: Iebcdf5cc881cad245643bb193760de35eb7f8b19
diff --git a/libc/arch-mips/bionic/__bionic_clone.S b/libc/arch-mips/bionic/__bionic_clone.S
index 11d596b..7b138ae 100644
--- a/libc/arch-mips/bionic/__bionic_clone.S
+++ b/libc/arch-mips/bionic/__bionic_clone.S
@@ -54,6 +54,9 @@
         j ra
 
 .L__thread_start_bc:
+        # Clear return address in child so we don't unwind further.
+        li      ra,0
+
         lw	a0,0(sp)	#  fn
         lw	a1,4(sp)	#  arg