[patch] Enable dual ABI for Filesystem TS

[Jonathan Wakely]

Currently the Filesystem TS library, libstdc++fs.a, only has symbols
for one ABI, whichever is configured as the default when GCC is built.

This adds dual ABI support, by building the relevant files twice, once
with each ABI (as we do already for libstdc++.so and libstdc++.a).

Tested powerpc64le-linux (both ABIs), committed to trunk.

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