[libc] Remove more header template files (#189066)
Get rid of several .h.def files which were used to ensure that the
macro definitions from llvm-libc-macro would be included in the public
header. Replace this logic with YAML instead - add entries to the
"macros" list that point to the correct "macro_header" to ensure it
would be included.
For C standard library headers, list several standard-define macros
to document their availability. For POSIX/Linux headers, only reference
a handful of macro, since more planning is needed to decide how to
represent platform-specific macro in YAML.
