Use gen-libm-test.py to generate ulps table for manual.

This patch extends gen-libm-test.py to generate the ulps table for the
manual, so meaning there is only a single ulps file parser needed and
another Perl script is eliminated.  As with the introduction of
gen-libm-test.py, this is designed to generate exactly the same
libm-err.texi as libm-err-tab.pl did.  (gen-libm-test.py is still
shorter in lines than the old gen-libm-test.pl even after this patch.)
Note that this introduces a Python dependency for building the manual,
which is thus noted in install.texi and NEWS.

Tested building html / info / pdf versions of the manual.

	* math/gen-libm-test.py: Import os.
	(ALL_FLOATS_MANUAL): New constant.
	(ALL_FLOATS_SUFFIX): Likewise.
	(Ulps.all_functions): New function.
	(real_all_ulps): Likewise.
	(generate_err_table_sub): Likewise.
	(generate_err_table): Likewise.
	(main): Handle -s and -m options.
	* manual/libm-err-tab.pl: Remove.
	* manual/Makefile ($(objpfx)stamp-libm-err): Use gen-libm-test.py
	instead of libm-err-tab.pl.
	[$(PERL) != no]: Change condition to [$(if $(PYTHON),$(PERL),no)
	!= no].
	* manual/install.texi (Tools for Compilation): Document
	requirement for Python to build manual.
	* INSTALL: Regenerated.