port 2P step 1: vendor CPython 2.7.18 as the mtpython static library

The embedded 40250 script layer needs its own interpreter on five platforms.
2.7 is end-of-life, so nothing can be fetched from the target SDKs and the
source is vendored (official 2.7.18 tarball, trimmed to 25MB; Lib/ stays for
step 3's python27.zip).

cpython-2.7.18/CMakeLists.txt builds one `mtpython` static library from exactly
the 133 objects the reference libpython2.7.a contains, off by default behind
-DMTGODOT_EMBED_PYTHON=ON. The source list and the built-in module table
(config/config.c, 39 entries) are shared; pyconfig.h is a probe result and is
not, so each platform keeps its own under config/<platform>/, regenerated by
tools/py_embed/gen_pyconfig.sh and checked against the shared table.

Three vendor patches, documented in docs/THIRD-PARTY.md: configure/configure.ac
learn arm64 on macOS, and posixmodule.c undefines the process-control calls it
hard-defines past pyconfig.h when building for iOS.

macOS (ctest python.embed: every builtin imports, codecs and pickle work off the
vendored Lib/), Android arm64 and iOS arm64 build. Linux needs a Linux host and
Windows needs a MinGW-vs-MSVC decision; both are noted for step 4.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

extension/third_party/cpython-2.7.18/Python/dynload_dl.c

@@ -0,0 +1,26 @@
+
+/* Support for dynamic loading of extension modules */
+
+#include "dl.h"
+
+#include "Python.h"
+#include "importdl.h"
+
+
+extern char *Py_GetProgramName(void);
+
+const struct filedescr _PyImport_DynLoadFiletab[] = {
+	{".o", "rb", C_EXTENSION},
+	{"module.o", "rb", C_EXTENSION},
+	{0, 0}
+};
+
+
+dl_funcptr _PyImport_GetDynLoadFunc(const char *fqname, const char *shortname,
+				    const char *pathname, FILE *fp)
+{
+	char funcname[258];
+
+	PyOS_snprintf(funcname, sizeof(funcname), "init%.200s", shortname);
+	return dl_loadmod(Py_GetProgramName(), pathname, funcname);
+}