Fix greenlet on aarch64 with gcc 4.8

GCC 4.8 has a fancy (though useless) method to save constant
loads.  It simply saves/restores the register known to have a
constant value around calls.  Which of course breaks the stack
switching scheme.  So make sure GCC doesn't know the return
value.
merge gcc 4.8 on aarch64 fixes

this is https://github.com/python-greenlet/greenlet/pull/29