This CL adds an instruction to the _start label that clears the frame
pointer. This allows stack walking code to determine when it has
reached the end of the stack.
The __bionic_clone function is similarly modified, for architectures
that weren't already doing both.
Test: bionic-unit-tests
Test: CtsBionicTestCases
Change-Id: Iea3949f52c44f7931f9fff2d60d4d9e5c742c120