SPARK-5669 [BUILD] [HOTFIX] Spark assembly includes incompatibly licensed libgfortran, libgcc code via JBLAS

Correct exclusion path for JBLAS native libs.
(More explanation coming soon on the mailing list re: 1.3.0 RC1)

Author: Sean Owen <sowen@cloudera.com>

Closes #4673 from srowen/SPARK-5669.2 and squashes the following commits:

e29693c [Sean Owen] Correct exclusion path for JBLAS native libs