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.

[libsel4] fix building with LibSel4FunctionAttributes

Correct exported functions that were still defined as 'static inline' to
use LIBSEL4_INLINE_FUNC so that they can be controlled by
LibSel4FunctionAttributes.

This is important to other languages that want to bind to the libsel4
static library instead of using the inline C functions.

riscv: Remove extranous lookupg definitions

This definitions don't really make sense as it is just PTs all the way down. Any resolution
that has >=SEL4_MAPPING_LOOKUP_NO_PT is a missing PT.

[SELFOUR-1156] RISC-V Port

Experimental release that supports both RV32 and RV64