riscv: Disambiguate between the C and C++ versions of isnan and isinf.

riscv: Disambiguate between the C and C++ versions of isnan and isinf.

When both the C and C++ versions are visible, the compiler will complain that
it doesn't know which one to use. By specifying the std namespace, it will
know to use the C++ version.

Change-Id: Ie1bbe1d95eadbad9644b4915c21f924d7d5c0b22
Reviewed-on: https://gem5-review.googlesource.com/4060
Reviewed-by: Alec Roelke <ar4jc@virginia.edu>
Maintainer: Alec Roelke <ar4jc@virginia.edu>