arch: x86: Allow disabling speculative store bypass

In order to mitigate against Spectre V4, add an option that will, at
boot time, verify if the CPU supports the SPEC_CTRL MSR; if so, it'll
attempt to disable the feature.

More information can be found in chapter 4 (Speculative Store Bypass
Mitigation) of the "Speculative Execution Side Channel Mitigations"
document, version 2, published by Intel: https://goo.gl/nocTcj



Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>