Details:
- Added preprocessor branches to test/3/test_gemm.c to explicitly
support row-stored matrices. Column-stored matrices are also still
supported (and is the default for now). (This is mainly residual work
leftover from initial integration of Eigen into the test drivers, so
if we ever want to test Eigen with row-stored matrices, the code will
be ready to use, even if it is not yet integrated into the Makefile
in test/3.)