libstdc++: Fix debug-mode tests for constexpr algorithms
These tests started failing at some point:
FAIL: 25_algorithms/copy/debug/constexpr_neg.cc (test for errors, line 49)
FAIL: 25_algorithms/copy/debug/constexpr_neg.cc (test for excess errors)
FAIL: 25_algorithms/equal/debug/constexpr_neg.cc (test for errors, line 47)
FAIL: 25_algorithms/equal/debug/constexpr_neg.cc (test for excess errors)
They only run with -D_GLIBCXX_DEBUG or make check-debug so seem to have
gone unnoticed until now.