replacing all ifndef with pargma once

All the kernel header files now use pargma once rather than the ifndef,
as the pre-processed C files do not change while header files
are protected with pargma once. This will also solve any naming issues
caused by ifndef.

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.

style: consistently align pointer with name

Run astyle with align-pointer=name

Remove many MODIFIES annotations.

These are redundant for any function which the C-to-Isabelle parser
actually analyses, which is now the vast majority of functions.

SELFOUR-632: implement cores non-architecture dependent structres

arm: Rearrange context switching code to make fastpath and regular code not duplicate functionality

Release snapshot