EDAC, MCE: Rework MCE injection

Add sysfs injection facilities for testing of the MCE decoding code.
Remove large parts of amd64_edac_dbg.c, as a result, which did only
NB MCE injection anyway and the new injection code supports that
functionality already.

Add an injection module so that MCE decoding code in production kernels
like those in RHEL and SLES can be tested.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>