[Patch] Fix glibc testing with GCC 4.9 compiler

[Florian Weimer]

* Steve Ellcey:

> I was testing glibc on a machine with GCC 4.9.2 and the tests died due
> to a warning in string/tester.c.  This routine has a DIAG_IGNORE for
> -Wmemset-transposed-args at GCC 5.0 and above but this warning is also
> given by my GCC 4.9.2 compiler.  This patch fixed the problem for me,
> is it OK to check it in?

I think this breaks the build for everyone with a 4.9 compiler which
does not support -Wmemset-transposed-args.  Wasn't this a
Debian-specific backport?