Perform build and magnum tuning

Building o//third_party/python now takes 5 seconds on my PC

This change works towards modifying Python to use runtime dispatching
when appropriate. For example, when loading the magnums in the socket
module, it's a good idea to check if the magnum is zero, because that
means the local system platform doesn't support it.

third_party/python/Python/makeopcodetargets.py

@@ -45,7 +45,7 @@ def main():
     if len(sys.argv) == 2:
         target = sys.argv[1]
     else:
-        target = "Python/opcode_targets.h"
+        target = "third_party/python/Python/opcode_targets.inc"
     with open(target, "w") as f:
         write_contents(f)
     print("Jump table written into %s" % target)