libstdc++: Do not declare strtok for C++26 freestanding (P2937R0)
This was recently approved for C++26.
We should define the __cpp_lib_freestanding_cstring macro in <string.h>
as well as <cstring>, but we do not currently install our own <string.h>
for most targets.
libstdc++-v3/ChangeLog:
* include/bits/version.def (freestanding_cstring): Add.
* include/bits/version.h: Regenerate.
* include/c_compatibility/string.h (strtok): Do not declare for
C++26 freestanding.
* include/c_global/cstring (strtok): Likewise.
* testsuite/21_strings/headers/cstring/version.cc: New test.