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.

arm: Rename plat_cleanInvalidateCache

Rename plat_cleanInvalidateCache to plat_cleanInvalidateL2Cache to
indicate that it is a L2 cache maintenance operation only.

Remove many MODIFIES annotations.

These are redundant for any function which the C-to-Isabelle parser
actually analyses, which is now the vast majority of functions.

add seL4_BenchmarkFlushCaches() for arm and ia32

Available when CONFIG_ENABLE_BENCHMARKS is set, flushes caches for
running cold-cache benchmarks

Release snapshot