[v3] Fix temporary switch to the "C" locale

[Benjamin Kosnik]

> BTW, rebuilding whole gcc tree seemed neccesarry to make check
> libstdc++ subdirs for the testcase. The step was,
> 1) patch libstdc++-v3 tree.
> 2) configure and build gcc tree, then install somewhere.
> 3) configure and build libstdc++-v3 using CC and CXX made in the
> previous step. 4) install libstdc++.so somewhere.
> 5) use LD_LIBRARY_PATH to point libstdc++.so made in the previous
> step. 6) make check patched libstdc++-v3 tree.
> just for information.

Another approach, assuming a current build directory from previous
unpatched state with g++, gcc, and libstdc++.

1) patch libstdc++ tree

2) cd (target)/libstdc++-v3; make clean

3) make all (from top level of build tree make all-target-libstdc++-v3
works)

4) make check