CMakeDetermineCompilerABI: Detect byte order as part of check

We already detect `sizeof(void*)`.  Detect the byte order as part of the
same check.

Issue: #21392

Help/variable/CMAKE_LANG_BYTE_ORDER.rst

@@ -0,0 +1,20 @@
+CMAKE_<LANG>_BYTE_ORDER
+-----------------------
+
+.. versionadded:: 3.20
+
+Byte order of ``<LANG>`` compiler target architecture, if known.
+If defined and not empty, the value is one of:
+
+``BIG_ENDIAN``
+  The target architecture is Big Endian.
+
+``LITTLE_ENDIAN``
+  The target architecture is Little Endian.
+
+This is defined for languages ``C``, ``CXX``, ``OBJC``, ``OBJCXX``,
+and ``CUDA``.
+
+If :variable:`CMAKE_OSX_ARCHITECTURES` specifies multiple architectures, the
+value of ``CMAKE_<LANG>_BYTE_ORDER`` is non-empty only if all architectures
+share the same byte order.