[libc] Make template_header optional for hdrgen (#127259)
This allows a YAML file to omit `template_header` and have no
`.h.def` file. A default template is generated based purely on
the information in the YAML file. This should handle most of the
cases. For now, it's exercised (aside from the hdrgen tests)
only for one of the simplest cases: <ctype.h>.
This includes making the parser notice the "standards" YAML field
at the top (header) level, not just in "functions" lists. The
standards listed for the header overall and for the individual
functions both feed into how a fully-generated header describes
itself in comments. To go with this, files using the default
generated template must stick to a new uniform set of spellings
for the "standards" lists. As more custom template files are
retired, the corresponding YAML files will need all their
standards lists normalized. For now, ctype.yaml is updated
with correct attribution for the POSIX `_l` extensions.
