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 attach return type

Add attach-return-type to astyle

style: correct parenthesis padding

Use astyle's unpad-paren to unpad all parentheses that are not included
by pad-header, pad-oper, and pad-comma.

Fix licenses (the rest)

SELFOUR-836: Hardware debug API functions should take tcb_t and not arch_tcb_t

Touches files all throughout x86 (32 and 64) and ARM.

s/D61/DATA61/ in license headers for consistency

Replace #pragma once with include guards

x64: Add x86_64 support