[v3] long double compat fixes for abi_check

[Benjamin Kosnik <bkoz at redhat dot com>]

My initial patch to fix this a week ago was wrong. I have higher hopes
for this patch, which I've tested by constructing various incorrect
libstdc++.so binaries, all of which were caught as expected. 

So, baseline symbol files with *_LDBL_* version names won't be
marked missing if the test symbol file doesn't contain any *_LDBL_*
versions. 

In addition, this adds a check for __float128 mangled names not in
*_LDBL_* version names.

I believe these are the last abi_check FAILs that involve
testsuite/util/* sources. The rest are configure/build issues.

In particular, this doesn't fix the issue that Jakub pointed out here:
http://gcc.gnu.org/ml/gcc-patches/2008-03/msg01262.html

where

baseline_dir="$glibcxx_srcdir/config/abi/post/${abi_baseline_pair}\$(MULTISUBDIR)"

isn't correct when --with-cpu=default32 on some platforms. 

I would like to fix this too, but haven't investigated this yet.

tested x86/linux

-benjamin

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