Use the kernel's sa_restorer for aarch64.

gdb was happy with what we had, but libgcc and libunwind weren't.
libgcc is happy with the kernel's restorer (because of the extra nop),
though libunwind looks like it's going to need code changes regardless.

We could make our restorer more like the kernel's one, but why bother
when we can just let the kernel supply the canonical one?

Bug: 17436734
Change-Id: I330fa5e68f23b1cf8133aa552896657b0b873ed3