UKSM: Remove custom memcmp

x86 memcmp can be moved to ../arch/x86/string*.h
We no need internal memcmp comparasion, if we can provide it to whole kernel

Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>
UKSM: merge is_full_zero into is_page_full_zero

Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>