[libstdc++] minor fixes and improvements to check-abi

[Benjamin Kosnik]

> I'm slightly confused why cross-targets would have been broken by the
> configury changes.  GLIBCPP_CONFIGURE_TESTSUITE now contains a test for
> native builds, but GLIBCPP_CONFIGURE_TESTSUITE itself is only run in the
> native build case.

It's bogus, don't get me wrong. I just didn't want to kill cross builds
before the weekend. It looks like the mips-elf cross doesn't know how to
link "C++" apps, as currently configured via libstdc++.

Here's the output log of the build failure:
http://gcc.gnu.org/ml/gcc-regression/2002-08/txt00125.txt

Here's the current mips-elf, "fixed":
http://people.redhat.com/geoffk/gcc-regression/mips-elf-lastbuild.txt.gz

It works because GLIBCCP_BUILD_ABI_CHECK is off by default. It has to be
turned on explicitly. As you've pointed out, this could be
unconditionally turned on in GLIBCPP_CONFIGURE_TESTSUITE as that macro,
itself, is only called when building natively.

If you can think of a better way to do this, please do.

-benjamin