Sanitizer tools desire having the vtables of any class with non-pure
virtual methods, even when none of them are called in the current
linkage unit. Work around this by making the affected classes pure
abstract and implementing them in a derived class in the respective
library responsible for creating them.

Bug swiftshader:31

Change-Id: I40046f605731eb1cc3825c1ede2d8d9b5826d0f5
Reviewed-on: https://swiftshader-review.googlesource.com/9914Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>