[4.3.0] remove system_error support

[Benjamin Kosnik]

After carefully reviewing the 4.3.0rc exports for libstdc++, I'm happy
with one exception.

Support for C++0x <system_error>, incomplete in 4.3.0, still has
potential for non-trivial changes going forward. Because this support
in libstdc++ includes exports in libstdc++.so, it is my preference to
pull support for the 4_3-branch, and keep what we have on the trunk,
allowing us the ability to allow this functionality to settle before it
is supported in an official GCC release.

The following patch against the 4_3-branch does exactly this.

tested x86_64/linux

-benjamin
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