x86/mce: Add an SPDX license identifiers

Add the SPDX tag and remove the GPL-v2 free text.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: <dmitriyz@google.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Tony Luck <tony.luck@intel.com>