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.

CMake: Refactor helpers to be included on demand

As more helper files get added it becomes more expensive to include them
all in common.cmake. We instead add them to the CMAKE_MODULE_PATH and
allow them to be imported as CMake modules. Adding global include guards
allows them to be imported multiple times without processing every time.

style: style cmake files consistently

CMake: Add custom polly toolchain and helpers

FindCustomPollyToolchain can be used to load a custom toolchain,
typically to be used as an argument to an ExternalProject_add call

CMake: Add cross_compiling helper script

Currently defines a function for creating a pkg-config executable with a
custom search path