bpo-40302: Add pycore_byteswap.h header file (GH-19552)

Add a new internal pycore_byteswap.h header file with the following
functions:

* _Py_bswap16()
* _Py_bswap32()
* _Py_bswap64()

Use these functions in _ctypes, sha256 and sha512 modules,
and also use in the UTF-32 encoder.

sha256, sha512 and _ctypes modules are now built with the internal
C API.

Modules/_testinternalcapi.c

@@ -9,6 +9,7 @@
 #define PY_SSIZE_T_CLEAN
 
 #include "Python.h"
+#include "pycore_byteswap.h"     // _Py_bswap32()
 #include "pycore_initconfig.h"   // _Py_GetConfigsAsDict()
 #include "pycore_gc.h"           // PyGC_Head
 
@@ -21,7 +22,7 @@ get_configs(PyObject *self, PyObject *Py_UNUSED(args))
 
 
 static PyObject*
-get_recursion_depth(PyObject *self, PyObject *args)
+get_recursion_depth(PyObject *self, PyObject *Py_UNUSED(args))
 {
     PyThreadState *tstate = PyThreadState_Get();
 
@@ -30,9 +31,38 @@ get_recursion_depth(PyObject *self, PyObject *args)
 }
 
 
+static PyObject*
+test_bswap(PyObject *self, PyObject *Py_UNUSED(args))
+{
+    uint16_t u16 = _Py_bswap16(UINT16_C(0x3412));
+    if (u16 != UINT16_C(0x1234)) {
+        PyErr_Format(PyExc_AssertionError,
+                     "_Py_bswap16(0x3412) returns %u", u16);
+        return NULL;
+    }
+
+    uint32_t u32 = _Py_bswap32(UINT32_C(0x78563412));
+    if (u32 != UINT32_C(0x12345678)) {
+        PyErr_Format(PyExc_AssertionError,
+                     "_Py_bswap32(0x78563412) returns %lu", u32);
+        return NULL;
+    }
+
+    uint64_t u64 = _Py_bswap64(UINT64_C(0xEFCDAB9078563412));
+    if (u64 != UINT64_C(0x1234567890ABCDEF)) {
+        PyErr_Format(PyExc_AssertionError,
+                     "_Py_bswap64(0xEFCDAB9078563412) returns %llu", u64);
+        return NULL;
+    }
+
+    Py_RETURN_NONE;
+}
+
+
 static PyMethodDef TestMethods[] = {
     {"get_configs", get_configs, METH_NOARGS},
     {"get_recursion_depth", get_recursion_depth, METH_NOARGS},
+    {"test_bswap", test_bswap, METH_NOARGS},
     {NULL, NULL} /* sentinel */
 };