libstdc++: Add std::time_get %r support [PR71367]

libstdc++: Add std::time_get %r support [PR71367]

This incremental patch adds std::time_get %r support (%p was added already
in the previous patch).  The _M_am_fm_format method previously in the header
unfortunately had wrong arguments and so was useless, so the largest
complication in this patch is exporting a new symbol in the right symbol
version.

2021-12-10  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/71367
	* config/locale/dragonfly/time_members.cc (_M_initialize_timepunct):
	Initialize "C" _M_am_pm_format to %I:%M:%S %p rather than empty
	string.
	* config/locale/gnu/time_members.cc (_M_initialize_timepunct):
	Likewise.
	* config/locale/generic/time_members.cc (_M_initialize_timepunct):
	Likewise.
	* include/bits/locale_facets_nonio.h (_M_am_pm_format): New method.
	* include/bits/locale_facets_nonio.tcc (_M_extract_via_format): Handle
	%r.
	* config/abi/pre/gnu.ver (GLIBCXX_3.4.30): Export _M_am_pm_format
	with const _CharT** argument, ensure it isn't exported in GLIBCXX_3.4.
	* testsuite/22_locale/time_get/get/char/71367.cc: New test.
	* testsuite/22_locale/time_get/get/wchar_t/71367.cc: New test.