Clear link register in __bionic_clone.

Since __bionic_clone uses tail-call to invoke __bionic_clone_entry,
at runtime the unwinder will reach the stack of the clone() function,
which belongs to the parent thread, if the link register is not cleared.

BUG: 14270816
Change-Id: Ia3711c87f8b619debe73748c28b9fb8691ea698e