Convert to SPDX license tags

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

trivial: Style changed files

libsel4vm: arch specific includes into arch subdir

Moved the architecture specific headers, being unique header files
that only exist for a given architecture, into an 'arch'
subdirectory. Since this library can be used across architectures
this avoids polluting the root libsel4vm namespace and adds
further structure to distinguish different interfaces.