x86/microcode/intel: Reuse find_matching_signature()

IFS uses test images provided by Intel that can be regarded as firmware.
An IFS test image carries microcode header with an extended signature
table.

Reuse find_matching_signature() for verifying if the test image header
or the extended signature table indicate whether that image is fit to
run on a system.

No functional changes.

Signed-off-by: Jithu Joseph <jithu.joseph@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Ashok Raj <ashok.raj@intel.com>
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
Link: https://lore.kernel.org/r/20221117035935.4136738-6-jithu.joseph@intel.com