hhvm binary has executable stack flag set

Build hhvm, then run `execstack -q path/to/hhvm`. If the output starts with `X`, as it does on my machine, the binary has the executable stack flag set. I see no reason why we need this, and if I clear the flag with `execstack -c` all tests pass.

I tried adding `.section .note.GNU-stack,"",@progbits` to all of our .S files but that didn't seem to fix it, so it's going to require some more digging.