[patch] libstdc++/60637 Fix C++98 std::signbit<long double>

[Jonathan Wakely]

This fixes PR60637 by using the appropriate built-in for the size of
the argument type. In Bugzilla Marc asked why we don't just use the
same code as for C++11, but I want to make this less intrusive change
on the branches (trunk is already OK anyway).

The new test is only run on x86 because it fails with a -Woverflow
warning on (at least) powerpc.

Tested x86_64-linux and powerpc64le-linux. Committed to gcc-5-branch
and gcc-4_9-branch.



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