Bug 1747970 - Move AC_CHECK_FUNC{,S} to python configure. r=firefox-build-system-reviewers,andi

- AC_HAVE_FUNCS is an alias to AC_CHECK_FUNCS, so it is covered.
- Nothing uses HAVE___CXA_DEMANGLE, so we don't explictly set it,
  although we do need the result of whether __cxa_demangle is supported.
- No moz.build uses MOZ_DEMANGLE_SYMBOLS, so we only `set_define` it.
- We leave dladdr in old-configure because it needs to move along other
  dl* things.
- The hotfix for AC_CHECK_FUNC is however not needed for dladdr, so we
  remove it.
- We replace the forced HAVE_LOCALECONV on Windows with a check shared
  with all platforms.

Differential Revision: https://phabricator.services.mozilla.com/D134858