Replace conform/list-header-symbols.pl with a Python script.

Continuing the move of test code from Perl to Python (which seems
uncontroversial, unlike dependencies on Python in the actual build of
glibc), this patch replaces conform/list-header-symbols.pl with a
Python script, as a first step in converting the conform/ tests.
(conform/glibcconform.py is an equivalent to GlibcConform.pm,
containing code that will be relevant to move than one of the conform/
scripts.)

Tested for x86_64, including verifying that the symbol lists generated
are identical to those generated by the Perl version.

	* conform/glibcconform.py: New file.
	* conform/list-header-symbols.py: Likewise.
	* conform/list-header-symbols.pl: Remove.
	* conform/Makefile (tests-special): Only add linknamespace tests
	if [PYTHON].
	($(linknamespace-symlists-tests)): Use list-header-symbols.py.