NEO_DISABLE_MITIGATIONS flag default should be true

[Shane McKee]

After discussion between Intel and Canonical’s security teams, we are in agreement that Spectre no longer needs to be mitigated for the GPU at the Compute Runtime level. At this point, Spectre has been mitigated in the kernel, and a clear warning from the Compute Runtime build serves as a notification for those running modified kernels without those patches. For these reasons, we feel that Spectre mitigations in Compute Runtime no longer offer enough security impact to justify the current performance tradeoff.

Intel themselves have enabled this flag in their builds available on their Github release page upstream.

PPA: https://launchpad.net/~mckeesh/+archive/ubuntu/lp2110131