PR libstdc++/78552 only construct std::locale for C locale once

Backport from mainline
2019-10-09  Jonathan Wakely  <jwakely@redhat.com>

	PR libstdc++/78552
	* src/c++98/locale_init.cc (locale::classic()): Do not construct a new
	locale object for every call.
	(locale::_S_initialize_once()): Construct C locale here.