Fix greenlet on aarch64 with gcc 4.8

GCC 4.8 has a fancy (though useless) method to save constant
loads on aarch64.  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.

This also fixes a (harmless) type error.  Testsuite of greenlet now
works on aarch64 with GCC 4.7 and 4.8, both when building
with -O2 and with -O0.