Implement C23 log10p1

C23 adds various <math.h> function families originally defined in TS
18661-4.  Add the log10p1 functions (log10(1+x): like log1p, but for
base-10 logarithms).

This is directly analogous to the log2p1 implementation (except that
whereas log2p1 has a smaller underflow range than log1p, log10p1 has a
larger underflow range).  The test inputs are copied from those for
log1p and log2p1, plus a few more inputs in that wider underflow
range.

Tested for x86_64 and x86, and with build-many-glibcs.py.

math/gen-tgmath-tests.py

@@ -744,6 +744,7 @@ class Tests(object):
         # TS 18661-4 functions.
         self.add_tests('exp10', 'r', ['r'])
         self.add_tests('log2p1', 'r', ['r'])
+        self.add_tests('log10p1', 'r', ['r'])
         self.add_tests('logp1', 'r', ['r'])
         # C23 functions.
         self.add_tests('fmaximum', 'r', ['r', 'r'])