bpo-38472: setup.py uses LC_ALL=C to check the C compiler (GH-30929)

Fix GCC detection in setup.py when cross-compiling. The C compiler is
now run with LC_ALL=C. Previously, the detection failed with a German
locale.
(cherry picked from commit a9503ac39474a9cb1b1935ddf159c0d9672b04b6)

Co-authored-by: Victor Stinner <vstinner@python.org>