kunit: cleanup assertion macro internal variables

All the operands should be tagged `const`.
We're only assigning them to variables so that we can compare them (e.g.
check if left == right, etc.) and avoid evaluating expressions multiple
times.

There's no need for them to be mutable.

Also rename the helper variable `loc` to `__loc` like we do with
`__assertion` and `__strs` to avoid potential name collisions with user
code.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>