Allow building for x86 Linux on GCC < 4.5

The noclone attribute is introduced in GCC 4.5, this patch allows building with older GCC versions (again). This may or may not reintroduce the segfaults that 5a0a628021357bf37cccbcc401e07dc99e9415ee speaks of, unfortunately I cannot get any segfaults with that commit's parent (nor does it describe a testcase), so I cannot verify the absence of introducing this particular segfault.

I can however confirm that after running a server with it (gevent on top of greenlet) I have had plenty of uncaught Python exceptions but not a single segfault (since early November).