lib: sbi_ipi: Process self-IPIs in sbi_ipi_send()

An IPI sent to the local hart can be processed directly instead of
triggering the IPI device. This is more efficient, and it avoids a
deadlock when the .sync callback is defined. Since interrupts are
disabled while handling an ecall, the IPI would not get delivered
until the next mret, but sbi_ipi_sync() is called before then.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Xiang W <wxjstz@126.com>