x86/mce: Get rid of the mce_severity function pointer

Turn it into a normal function which calls an AMD- or Intel-specific
variant depending on the CPU it runs on.

No functional changes.

Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://lkml.kernel.org/r/20210922165101.18951-2-bp@alien8.de