convert license headers to SPDX

Includes license review of 3rd-party files, adding missing upstream
headers, and moving the UNSW OZPLB license into its own file rather
than replicating it in the header.

elfloader: Support leaving aarch64 EL2 to aarch32

This allows aarch32 seL4 kernel configurations to be loaded from a
64bit entry point. When this setting is configured, an additonal
symbol, _start32, is created that is still a valid 32bit entry point.