Force no default function when using CUDA

[stephen-hqxu]

The compiler emits a ton of warnings when using CUDA. An snippet of the warning message looks like this:

type_vec4.hpp(101): warning : __device__ annotation is ignored on a function("vec") that is explicitly defaulted on its first declaration
type_vec4.hpp(101): warning : __host__ annotation is ignored on a function("vec") that is explicitly defaulted on its first declaration

For CUDA, a trivial default function already tells both host and device compilers to emit code, therefore __device__ and __host__ qualifiers are ignored. In another word, function declared as default implies __device__ __host__ qualifier.

As a resolution, either the function qualifiers should be removed, or force to not use default function. Considering the changes made in #1106, I picked the second choice.

[christophe-lunarg]

Thanks for contributing!