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C23 adds various <math.h> function families originally defined in TS 18661-4. Add the rootn functions, which compute the Yth root of X for integer Y (with a domain error if Y is 0, even if X is a NaN). The integer exponent has type long long int in C23; it was intmax_t in TS 18661-4, and as with other interfaces changed after their initial appearance in the TS, I don't think we need to support the original version of the interface. As with pown and compoundn, I strongly encourage searching for worst cases for ulps error for these implementations (necessarily non-exhaustively, given the size of the input space). I also expect a custom implementation for a given format could be much faster as well as more accurate, although the implementation is simpler than those for pown and compoundn. This completes adding to glibc those TS 18661-4 functions (ignoring DFP) that are included in C23. See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118592 regarding the C23 mathematical functions (not just the TS 18661-4 ones) missing built-in functions in GCC, where such functions might usefully be added. Tested for x86_64 and x86, and with build-many-glibcs.py.