Fix invalid escape sequence in build-many-glibcs.py

Running build-many-glibcs.py with Python 3.12 or later produces a
warning:

build-many-glibcs.py:173: SyntaxWarning: invalid escape sequence '\.'
  m = re.fullmatch('([0-9]+)\.([0-9]+)[.0-9]*', l)

Use a raw string instead to avoid that warning.  (Note: I haven't
checked whether any other Python scripts included with glibc might
have issues with newer Python versions.)