x86/cpu: Use cpu_has_sep for configuring the SYSENTER MSRs

x86/cpu: Use cpu_has_sep for configuring the SYSENTER MSRs

Currently, configuration of the SYSENTER MSRs are behind a vendor check for
Intel and Centaur, but this misses Zhaoxin.

Use the feature bit, rather than a vendor check.  cpu_has_sep is cleared early
for AMD processors, which can't use SYSENTER/SYSEXIT when operating in long
mode.

Suggested-by: Jan Beulich <JBeulich@suse.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>