Slightly reworked patch.  I plan to apply this shortly.

- Use ~(_MCW_PC | _MCW_RC) rather than (_MCW_DN | ...), since this seems more future proof:  there's a possibility that more flags could be added later.

- Put the usual do { ... } while (0) around the _Py_SET_53BIT_PRECISION_END

- Make it clear that we don't care about the return value of the second two __control87_2 patches.

- Minor style fixes (e.g., spaces around a "bitwise and" ampersand to help distinguish it from an "address of" ampersand).


I'm not too worried about the fenv_access pragma:  it seems that the main thing it would guard against is compile-time folding and evaluation of expressions, where the compiler isn't taking the possibility of control word changes into account.  As far as I can tell, we shouldn't really care about this, since at the time that Python itself is compiled, the control word is likely to be what we want.