Given that spinning is only checked once at the start of a
_cygtls::interrupt_now() which is called in a loop, it's probably
not necessary to mark _cygtls::spinning as volatile.
However, spinning is changed from assembler code and we don't
want the compiler to make funny assumptions, so, better safe than
sorry.
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>
3a9fb7c561