bpo-45412: Move _Py_SET_53BIT_PRECISION_START to pycore_pymath.h (GH-28882)

Move the following macros , to pycore_pymath.h (internal C API):

* _Py_SET_53BIT_PRECISION_HEADER
* _Py_SET_53BIT_PRECISION_START
* _Py_SET_53BIT_PRECISION_END

PEP 7: add braces to if and "do { ... } while (0)" in these macros.

Move also _Py_get_387controlword() and _Py_set_387controlword()
definitions to pycore_pymath.h. These functions are no longer
exported.

pystrtod.c now includes pycore_pymath.h.