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.

Remove symlinked libsel4 files from include dir

These files can be included normally using libsel4 include paths. This
removes situations where the same file is available under different
include paths due to symlinking into different directory structures.

style: consistently attach return type

Add attach-return-type to astyle

s/DEBUG/CONFIG_DEBUG_BUILD/

DEBUG definition is not supposed to be used in the kernel, rather CONFIG_DEBUG_BUILD,
which can be toggled separately to user notion of DEBUG

Fix type error in benchmark.h

Remove circular include between debug and benchmark_track

benchmark_track.h does not depend upon anything in debug.h and
so it does not need to include it. Once this include is removed
debug.h can pull its definition of ksKernelEntry from benchmark_track.h

Fix benchmark_debug_syscall_start

The lowest syscall number is -8, which doesnt' fit in 3 bits -> change
to 4.
Also, we were storing the -ve number, which also doesn't fit into 3
bits -> invert it when we store it.

Benchmark: Pack arch-independent benchmark-related files into separate directories