Only load a dependent library when it has an expected symbol.

Bug 18752589

Change-Id: Ic42be261dc101810e6363c4823620d6c827c7975
Reviewed-on: https://swiftshader-review.googlesource.com/2880
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>