MCE: use new common mce handler on AMD CPUs

MCE: use new common mce handler on AMD CPUs

Factor common machine check handler out of intel specific code
and move it into common files.
Replace old common mce handler with new one and use it on AMD CPUs.
No functional changes on Intel side.
While here fix some whitespace nits and comments.

Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>
Committed-by: Keir Fraser <keir@xen.org>