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.

x86: Move xsetbv to general header

The xsetbv and write_xcr0 wrapper are not 32-bit specific and were
placed in the wrong location originally.

x86: Add functions for writing to the extended control registers

x86: Remove underscores from symbols as they are in violation of the C standard

x86: Make parts of what have become 32-bit specific headers common

The reason these were not made common to begin with is so that there
was a commit that was just a rename of these files to make any merges
with other branches that might exist easier

x86: Minimal changes to fix previous commit that renamed headers