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.

riscv32 does not support huge pages

For some reason the kernel documented and maintained constants for a
512MiB 'huge page' in riscv32 which is not part of the specification.

The references and constants are removed

SELFOUR-161: Merge Page_Remap with Page_Map

- Remove Remap function from seL4 API for arm, x86, riscv and the
respective invocation implementation.
- Update Map as replacement for Remap
- Update manual

This allows a change of rights if the frame being mapped is already
mapped in at the given vaddr. To map a page to a different address,
unmap it first.

Co-authored-by: Hesham Almatary <hesham.almatary@data61.csiro.au>
Co-authored-by: Anna Lyons <Anna.Lyons@data61.csiro.au>
Co-authored-by: Victor Phan <Victor.Phan@data61.csiro.au>
Co-authored-by: Kent McLeod <Kent.Mcleod@data61.csiro.au>

manual: clarify VSpace as top-level paging structure

This change updates the vspace chapter to separate the high-level concept of a VSpace from the
architectually defined objects. It also updates the various names for the vspace parameter to all be
vspace.

manual: refactor vspace chapter

- instead of referring to only PDs and PTs, use generic language about paging structures
- link to each architectures methods
- create tables that show the info for all archictures

manual: remove hard-coded PD, PT and Page invocations

- increases the maintainability of the docs
- move descriptions of methods to the API reference
- remove hardcoded invocations in manual

This commit removes the content completely from vspace.tex, as it is out of date and needs
restructuring. The next commit does the restructuring.

manual: remove hard-coded ASID invocations

- move documentation to API ref
- create tables pointing to the API ref for these objects

riscv: document page table structure

Manual: vspace.tex - document cache operations on page directories

manual: Correct ia32 to x86 in some sections

SELFOUR-254: manual: describe mapping attributes

Correctly describes the fact that on ARM mapping attributes are
a set of flags that can be OR'd together, and on x86 they are an
enumeration of possible values

manual: Purely cosmetic changes / improved wording

further support for XN bit on ARM

Release snapshot