x86: add a synthetic TSC_RELIABLE feature bit

Impact: None, bit reservation only

Add a synthetic TSC_RELIABLE feature bit which will be used to mark
TSC as reliable so that we could skip all the runtime checks for
TSC stablity, which have false positives in virtual environment.

Signed-off-by: Alok N Kataria <akataria@vmware.com>
Signed-off-by: Dan Hecht <dhecht@vmware.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>