libstdc++: Fix Python scripts to output the correct filename

These scripts both print "generated by $file, do not edit" header but
one of them prints the wrong filename. Use the built-in __file__
attribute to ensure it's correct.

contrib/ChangeLog:

	* unicode/gen_libstdcxx_unicode_data.py: Fix header of generated
	file to name the correct script.

libstdc++-v3/ChangeLog:

	* include/bits/text_encoding-data.h: Regenerate.
	* include/bits/unicode-data.h: Regenerate.
	* scripts/gen_text_encoding_data.py: Fix header of generated
	file to name the correct script.