libstdc++: Do not declare strtok for C++26 freestanding (P2937R0)

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.