[v3] tweak -ffunction-sections configure test

[Brian Dessent]

Benjamin Kosnik wrote:

> This is the behavior I was expecting on mingw32. I'm not quite sure why
> you think this is in error.
> 
> The end result should be that -ffunction-sections is not used when
> compiling the files in libstdc++-v3/src on this target.

Oh, okay.  Then it makes more sense. :) 

Is the reason for disabling it due to the warning causing spurious FAILs
in the testsuite (or halting the build when -Werror is turned on), or
because there's actually some problem with the library built with
-ffunction-sections?

Brian