Decentralize Python native module linkage

We can now link even smaller Python binaries. For example, the hello.com
program in the Python build directory is a compiled linked executable of
hello.py which just prints hello world. Using decentralized sections, we
can make that binary 1.9mb in size (noting that python.com is 6.3 megs!)

This works for nontrivial programs too. For example, say we want an APE
binary that's equivalent to python.com -m http.server. Our makefile now
builds such a binary using the new launcher and it's only 3.2mb in size
since Python sources get turned into ELF objects, which tell our linker
that we need things like native hashing algorithm code.

third_party/python/Lib/os.py

@@ -1158,7 +1158,10 @@ def popen(cmd, mode="r", buffering=-1):
         raise ValueError("invalid mode %r" % mode)
     if buffering == 0 or buffering is None:
         raise ValueError("popen() does not support unbuffered streams")
-    import subprocess, io
+    try:
+        import subprocess, io
+    except ImportError:
+        raise ImportError('please use subprocess module')
     if mode == "r":
         proc = subprocess.Popen(cmd,
                                 shell=True,