[libc] Make math-macros.h C++-friendly (#86206)

The isfinite, isnan, and isinf "functions" are specified by C99..C23 to
be macros that act as type-generic functions. Defining them as their
__builtin_* counterparts works fine for this. However, in C++ the
identifiers need to be usable in different contexts, such as being
declared inside a C++ namespace. So define inline constexpr template
functions for them under `#ifdef __cplusplus`.