lib: Allow sending IPI to self.

S-mode software may send IPI to self. For example,
tlbflush may be executed for the same hart.

Let the hart send IPI to itself. It's an overhead
but doesn't break anything. However, if we don't
allow it, it breaks if S-mode keep sending IPIs.

Signed-off-by: Atish Patra <atish.patra@wdc.com>