libstdc++: Fix orphaned/nested output of configure checks

This moves two AC_MSG_RESULT lines for <uchar.h> features so that they
are only printed when the corresponding AC_MSG_CHECKING actually
happened. This fixes configure output like:

checking for uchar.h... no
no
checking for int64_t... yes

Also move the AC_MSG_CHECKING for libbacktrace support so it doesn't
come after AC_CHECK_HEADERS output. This fixes:

checking whether to build libbacktrace support... checking for sys/mman.h... (cached) yes
yes

libstdc++-v3/ChangeLog:

	* acinclude.m4 (GLIBCXX_CHECK_UCHAR_H): Don't use AC_MSG_RESULT
	unless the AC_MSG_CHECKING happened.
	* configure: Regenerate.