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: Set up the idle threads' per-core stacks

Use per-core stacks for idle thread stacks.

Remove platform IPC buffer register.

This removes the assumption that each platform sotres the IPC buffer
address in a platform-specific register. The IPC buffer address is
instead stored in a thread-local variable in libsel4 which must be
initialised by the runtime.

Consistent naming of FaultIP and NextIP in kernel

Always refer to the virtual register that stores the address of a fault
as FaultIP and the register that stores the return for a fault NextIP.

RISC-V: Use correct sstatus value for idle thread

We were incorrectly enabling interrupts for the current context instead
of the return context. This was causing the hart to be preempted before
an sret call could be performed when switching to the idle thread.

style: consistently attach return type

Add attach-return-type to astyle

Remove attribute for RISCV as well

riscv: Correct stack setting for idle thread

This sets the idle thread to have a stack top, instead of a stack bottom, which means
it no longer needs to have a custom optimize pragma to avoid using the stack.

riscv: remove HARTID

[SELFOUR-1156] RISC-V Port

Experimental release that supports both RV32 and RV64