Libstdc++: Fix bootstrap failure for cross without tm.tm_zone [PR119550]

Libstdc++: Fix bootstrap failure for cross without tm.tm_zone [PR119550]

In r15-8491-g778c28c70f8573 I added a use of the Autoconf macro
AC_STRUCT_TIMEZONE, but that requires a link-test for the global tzname
object if tm.tm_zone isn't supported. That link-test isn't allowed for
cross-compilation, so bootstrap fails if tm.tm_zone isn't supported.

Since libstdc++ only cares about tm.tm_zone and won't use tzname anyway,
we don't need the link-test. Replace AC_STRUCT_TIMEZONE with a custom
macro that only checks for tm.tm_zone. We can improve on the Autoconf
macro by checking it's a suitable type, which isn't actually checked by
AC_STRUCT_TIMEZONE.

libstdc++-v3/ChangeLog:

	PR libstdc++/119550
	* acinclude.m4 (GLIBCXX_STRUCT_TM_TM_ZONE): New macro.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Use GLIBCXX_STRUCT_TM_TM_ZONE.
	* include/bits/chrono_io.h (__formatter_chrono::_M_c): Check
	_GLIBCXX_USE_STRUCT_TM_TM_ZONE instead of
	_GLIBCXX_HAVE_STRUCT_TM_TM_ZONE.