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

[Joseph Myers]

On Mon, 12 Oct 2015, Bernd Schmidt wrote:

> A similar sequence of tests also occurs for *-aix*. I don't suppose the
> function is likely to exist there or on other non-glibc targets?

Given that the case there has "# We don't yet support AIX's TLS ABI." and 
GCC_CHECK_TLS commented out, I don't think this function (which is 
concerned with support for destructors of C++11 thread_local variables) is 
of any current relevance to that case.

-- 
Joseph S. Myers
joseph@codesourcery.com