Control for Special Register Buffer Data Sampling mitigation.

New microcode update for Intel enables mitigation for SRBDS, which
slows down RDSEED and related instructions. The update also provides
a control to limit the mitigation to SGX enclaves, which should
restore the speed of random generator by the cost of potential
cross-core bufer sampling.

See https://software.intel.com/security-software-guidance/insights/deep-dive-special-register-buffer-data-sampling

GIve the user control over it.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D25221