[C++ verification] update isnan OM

[XLiZHI]

Follow suggestion in https://github.com/esbmc/esbmc/pull/2008#issuecomment-2318404541

[XLiZHI]

I believe we'll also need isnanf and isnanl. That's what the CIMPORT1 macro has been for.

Hi @fbrausse, What I learned was __builtin_isnan, which handles all types. isnanf and isnanl exist for compatibility reasons; compilers other than clang and gcc do not support this built-in function.

[lucasccordeiro]

Thanks for submitting this PR, @XLiZHI.