Only a few modules are involved in the bootstrap. The filename is
mostly used to display in the traceback. There is already a fallback
in the traceback-printing code that tries to look through sys.path for
a file matching the module if it can't open the filename from the code
object.

So I suggest to do something much simpler -- if the default encoding
isn't set yet, use ASCII + replace, and leave it at that -- don't
bother fixing these things later.

Another thought: when installed, the built-in modules are compiled to
byte code by compileall.py, which runs late enough so that the
filesystem encoding is known and everything can be done right.

I have a question for Alexandre related to frozen.c -- why is there a
mode line with an encoding involved in freezing hello.py?