libstdc++: testsuite: work around bitset namespace pollution
rtems6 declares a global struct bitset in a header file included
indirectly by sys/types.h, that ambiguates the unqualified references
to bitset after "using namespace std" in the testsuite.
Work around the namespace pollution with using declarations of
std::bitset.
for libstdc++-v3/ChangeLog
* testsuite/23_containers/bitset/cons/dr1325-2.cc: Work around
global struct bitset.
* testsuite/23_containers/bitset/ext/15361.cc: Likewise.
* testsuite/23_containers/bitset/input/1.cc: Likewise.
* testsuite/23_containers/bitset/to_string/1.cc: Likewise.
* testsuite/23_containers/bitset/to_string/dr396.cc: Likewise.