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.