Makefile: support flag -fsanitizer-coverage=trace-cmp

Makefile: support flag -fsanitizer-coverage=trace-cmp

The flag enables Clang instrumentation of comparison operations
(currently not supported by GCC).  This instrumentation is needed by the
new KCOV device to collect comparison operands.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Victor Chibotaru <[email protected]>
Signed-off-by: Alexander Potapenko <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Andrey Konovalov <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Alexander Popov <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Vegard Nossum <[email protected]>
Cc: Quentin Casasnovas <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>