arch: added generic vector register

arch: added generic vector register

This commit adds a new generic vector register to have a cleaner
implementation of SIMD ISAs.

Nathanael's idea, Rekai's implementation.

Change-Id: I60b250bba6423153b7e04d2e6988d517a70a3e6b
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2704
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Reviewed-by: Tony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>