x86/mce: clear MSR_IA32_MCG_STATUS by writing 0

x86/mce: clear MSR_IA32_MCG_STATUS by writing 0

On Intel CPU, an attemp to write to MSR_IA32_MCG_STATUS with any
non-zero value would result in #GP.

This commit writes 0 on AMD CPU as well instead of just clearing MCIP
bit, because all non-reserved bits of MSR_IA32_MCG_STATUS have been
handled at this point.

Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>