[libc] Add merge_yaml_files feature to hdrgen (#127269)
This allows a sort of "include" mechanism in the YAML files. A
file can have a "merge_yaml_files" list of paths (relative to the
containing file's location). These are YAML files in the same
syntax, except they cannot have their own "header" entry. Only
the lists (types, enums, macros, functions, objects) can appear.
The main YAML file is then processed just as if each of its lists
were the (sorted) union of each YAML file's corresponding list.
This will enable maintaining a single source of truth for each
function signature and other such details, where it is necessary
to generate the same declaration in more than one header.
