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.

mcs: scheduling context donation over ipc

After this commit, threads blocked on an endpoint can recieve a
scheduling context from the thread that wakes the blocked thread.

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.

style: set code width to 120

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.

libsel4: fix licenses

- some were incorrectly marked GPL (libsel4 is BSD)
- update NICTA --> DATA61 etc
- fix tags D61 --> DATA61
- update year to 2017

s/D61/DATA61/ in license headers for consistency

SELFOUR-675: x64: Increase message registers from 2 to 4

Replace #pragma once with include guards

[STYLE_FIX]

x64/libsel4: Add x64 libsel4 implementation