Test for __cxa_thread_atexit_impl when cross-compiling libstdc++ for GNU targets.

Test for __cxa_thread_atexit_impl when cross-compiling libstdc++ for GNU targets.

I noticed that when testing glibc with a cross compiler I got

UNSUPPORTED: nptl/tst-thread_local1

because the libstdc++-v3 configuration for cross compiling defaulted
to __cxa_thread_atexit_impl not being available.  This patch fixes
GLIBCXX_CROSSCONFIG to run the same test (for the case covering
targets with glibc) for __cxa_thread_atexit_impl as for native
compilation, just as it runs most of the other tests done for native
compilation (for these targets, it's not possible to build libstdc++
without already having built libc, so link tests are OK).

Tested with no regressions for cross to arm-none-linux-gnueabi.

	* crossconfig.m4 (GLIBCXX_CROSSCONFIG) <*-linux* | *-uclinux* |
	*-gnu* | *-kfreebsd*-gnu | *-knetbsd*-gnu | *-cygwin*>: Check for
	__cxa_thread_atexit_impl.
	* configure: Regenerate.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@228695 138bc75d-0d04-0410-961f-82ee72b054a4