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.

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