retpoline/module: add bit defs for use_ibpb

retpoline/module: add bit defs for use_ibpb

Replace bit references for IBPB state in use_ibpb with:

    SPEC_CTRL_IBPB_INUSE
    SPEC_CTRL_IBPB_SUPPORTED
    SPEC_CTRL_IBPB_ADMIN_DISABLED

Change comment block describing use_ibrs flags to use symbolic values.
Remove duplicate comment block describing flag values in use_ibrs and use_ibpb.
Cosmetic change to defs for sysctl_ibrs_enabled, sysctl_ibpb_enabled and
sysctl_lfence_enabled in preparation for adding additional sysctl_* values.

Orabug: 27457549

Signed-off-by: Chuck Anderson <chuck.anderson@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>
Conflicts:
	arch/x86/kernel/cpu/spec_ctrl.c