[libc][math] Extend iscanonical macro to _Float16 and float128 (#204981)

iscanonical is a C23 type-generic macro, so the f16/f128 variants are
surfaced through it rather than as functions in the generated math.h.
float128 is only listed when distinct from long double (LDBL_MANT_DIG !=
113) to avoid two _Generic associations with compatible types.

Stacked below: https://github.com/llvm/llvm-project/pull/205016

Co-authored-by: lntue <lntue@google.com>
GitOrigin-RevId: 5aa651511dcf92376af21c7029e6cd4abef194dd