riscv: Improve IPI code

The removed while loop in ipi_send_target can cause a deadlock, if
the target of an IPI remote call enters handleIPI through the
clh_is_ipi_pending path in smp/lock.h.

Signed-off-by: Yanyan Shen <yanyan.shen@data61.csiro.au>

Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

riscv: Add IPI calls and handlers

Add IPI call functions and handler functions.