TSX Asynchronous Abort mitigation for Intel CVE-2019-11135.

TSX Asynchronous Abort mitigation for Intel CVE-2019-11135.
This CVE has already been announced in FreeBSD SA-19:26.mcu.

Mitigation for TAA involves either turning off TSX or turning on the
VERW mitigation used for MDS. Some CPUs will also be self-mitigating
for TAA and require no software workaround.

Control knobs are:
machdep.mitigations.taa.enable:
        0 - no software mitigation is enabled
        1 - attempt to disable TSX
        2 - use the VERW mitigation
        3 - automatically select the mitigation based on processor
	    features.

machdep.mitigations.taa.state:
        inactive        - no mitigation is active/enabled
        TSX disable     - TSX is disabled in the bare metal CPU as well as
                        - any virtualized CPUs
        VERW            - VERW instruction clears CPU buffers
	not vulnerable	- The CPU has identified itself as not being
			  vulnerable

Nothing in the base FreeBSD system uses TSX.  However, the instructions
are straight-forward to add to custom applications and require no kernel
support, so the mitigation is provided for users with untrusted
applications and tenants.

Reviewed by:	emaste, imp, kib, scottph
Sponsored by:	Intel
Differential Revision:	22374