libstdc++: Correct export checks for _GLIBCXX_EXTERN_TEMPLATE

Replace a basic_string specific _GLIBCXX_EXTERN_TEMPLATE > 0 with
simple check for _GLIBCXX_EXTERN_TEMPLATE. This make sure that
extern definition is also use for -O0 (default).
Exported declarations are also important for future extensibility,
as they always pick latest implementation.

libstdc++-v3/ChangeLog:

	* include/std/format (__do_vformat_to): Change the preprocessor
	check for __do_vformat_to.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
(cherry picked from commit 6ac3a6c8aa22dc78f68eb3c99e2c600ca3e57904)