fenv: Use slow path in C++

[aokblast]

Summary

C++ exposes cfenv functions svia using ::func. Our name-mangling
mechanism rewrites all function calls causing symbols such as
std::feclearexcept to be transformed into std::__feclearexcept_int.
Since no such function exists, compilation fails.

The using ::feclearexpect declarations themselves are unaffected because
they are not function calls, which further exposes the mispatch

As a result, enable the fast path only for C and fall back to the slow
path in C++.