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.

RFC-3: Update context for x86 to use FS and GS.

TLS_BASE virtual register is replaced with FS_BASE and GS_BASE virtual
registers.

The FS_BASE and GS_BASE virtual registers are moved to the end of the
context so they need not be considered in the kernel exit and entry
implementation.

Removed tracking of ES, DS, FS, and GS segment selectors on kernel entry
and exit.

ES and DS are clobbered on kernel entry with the RPL 3 selector for a
DPL 3 linear data segment.

FS is clobbered on exit with the RPL 3 selector for the DPL 3 segment
with FS_BASE as the base. This is done on exit to reload the value from
the GDT.

GS is clobbered on exit with the RPL 3 selector for the DPL 3 segment
with GS_BASE as the base. This is done on exit to reload the value from
the GDT.

Kernel entry and exit code is refactored, simplified, and improved in
light of the above changes.

x64: update verified config to use fsgsbase instr

The verification platform for x64 relies on the fsgsbase instruction.

More standard constant name, moved ASID constants to arch generic files

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

style: consistently align pointer with name

Run astyle with align-pointer=name

x86_64: Use sys[ret/exit]q instead of rex prefix

Clang does not support the rex.w instruction prefix and instead requires
sys[ret/exit] mnemonics.

x64: SKIM window to mitigate Meltdown (CVE-2017-5754) on x86-64

Introduces a kernel option that, when enabled, reduces the kernel window in a user address
space to just be Static Kernel Image and Microstate (SKIM), instead of the full kernel
address space. This isolates the important kernel data from the user preventing a
Meltdown style attack being able to violate secrecy. The kernel text and read only data,
i.e. anything that is static from boot, is not secret and can be allowed in the SKIM window
and potentially read by the user. Additionally to switch to and from the actual kernel
address space a small amount of state needs to also be in the SKIM window.

This is only an implementation for x86-64, although the same design is applicable to ia32

x86: Separate definition for kernel vspace root

Introduces a separate definition for vspace root that the kernel runs on. Having this
be distinct from the global vspace root allows for potential future distinction
between the global root (that is copied into all user address spaces) and the address
space that the kernel runs in.

x64: Rename global paging structures to kernel

Currently the kernel address space and paging structures are used as the global ones.
This commit renames the paging structures from Global to Kernel to reflect this and
allows for separate global structures to be introduced in the future.

x86: Return instead of failing in mode cap functions

Verification will guarantee that all usages of the parent cap_get* functions will
be correct and that this case does not happen. However, it is difficult to prove
that the `fail` cannot happen in isolation, and is ultimately not neccessary.

SELFOUR-1062: Hide all VT-x related code behind #ifdef guards

The VT-x implementation is not going to be verified at the moment, and so the code for
it needs to be hidden from verification, which we do by #ifdef'ing it out if the VT-x
is not enabled. As a result the VT-x configuration depends on a non verification target

Fix licenses (the rest)

SELFOUR-879: expose index and entry constants

Fix: add pdpt stuff removed from shared x86 code to x86_64

s/D61/DATA61/ in license headers for consistency

x64: VT-x related cap and object definitions

* Adds object and cap definitions for VT-x structures (VCPU and EPT).
* Extends the asid_map implementation to support ASIDs in the EPT
* Adds size definitions for VCPU and EPT objects

x86: Refactor tlb_bitmap to be mode generic

Refactors the TLB bitmap code to be generic across ia32 and x86-64.

x86: Refactor generic x86 pte/pde manipulation

Previously generic functions for manipulating page table and page
directory entries were placed in mode/structures.h. These are moved
to a mode/vspace.h instead so that they can use existing functions
defined in arch/vspace.h.

The x86_make_pde_mapping function is dropped in this move as it is
not used. Instead it is replaced with a function for creating an
empty mapping for whatever is the vspace root.

x64: Match ASID changes on ia32

Replace #pragma once with include guards

[STYLE_FIX]

x64: Add x86_64 support