1.0.25.24: x86/x86-64 runtime pseudo atomic fixes

1.0.25.24: x86/x86-64 runtime pseudo atomic fixes

Make sure that {clear,set}_pseudo_atomic_{atomic,interrupted} boil
down to a single assembly intruction. And that the compiler does not
reorder them.

I think the ppc bits in pseudo-atomic.h are still broken on both
accounts.