PR libstdc++/68737 Do not use vsnprintf on HPUX

PR libstdc++/68737 Do not use vsnprintf on HPUX

It doesn't conform to the spec, so use vsprintf with a large buffer
instead.

	PR libstdc++/68737
	* config/locale/generic/c_locale.h (__convert_from_v)
	[_GLIBCXX_USE_C99_STDIO]: Also check _GLIBCXX_HAVE_BROKEN_VSNPRINTF.
	* config/os/hpux/os_defines.h: Define _GLIBCXX_HAVE_BROKEN_VSNPRINTF.
	* include/bits/locale_facets.tcc (num_put::_M_insert_float)
	[_GLIBCXX_USE_C99_STDIO]: Also check _GLIBCXX_HAVE_BROKEN_VSNPRINTF.

(cherry picked from commit c98f255154798847bdd1fc6ce33266c1a1ddc13a)