POWER: Add Dgemm kernel for POWER processor (#9459)
* POWER: Add Dgemm kernel for POWER processor
This patch adds new dgemm kernel specific to POWER processor.
* POWER: Restrict new functions to VSX in header
* Remove warning check in header
* POWER: Dgemm Adjust indentation
Fixing indentation based on review comments.
Co-authored-by: Rajalakshmi Srinivasaraghavan <rajis@linux.ibm.com>
