KCFI: Allow distinguishing indirect functions with otherwise identical signature

[kees]

https://lore.kernel.org/lkml/20220211133803.GV23216@worktop.programming.kicks-ass.net/

Andrew put in the request for __attribute__((cfi_seed(blah)))
to allow distinguishing indirect functions with otherwise identical
signature; eg. cookie = hash32(blah##signature)

[kees]

CC @samitolvanen