checkasm: Explicitly declare function prototypes

Now we no longer have to rely on function pointers intentionally
declared without specified argument types.

This makes it easier to support functions with floating point parameters
or return values as well as functions returning 64-bit values on 32-bit
architectures. It also avoids having to explicitly cast strides to
ptrdiff_t for example.

Signed-off-by: Anton Khirnov <anton@khirnov.net>