Fix number of block columns to NOT overflow the cache (PowerPC) abnormally in GEMV

Chip Kerchnerrequested to merge
ChipKerchner/eigen:level2_blocksize_MR into master

Fix number of block columns to NOT overflow the cache (PowerPC) abnormally in GEMV. Timings were unusual when number of columns was between 2500-3200-ish.

Edited by Chip Kerchner

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