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.

Pass the target triple through when using clang

Whenever clang is invoked in a custom setting (not generated by CMake),
make sure to pass the triple as a flag

Allow building CPIO with no files

MakeCPIO will now always produce a CPIO even when an empty list of files
is specified.

cpio: fix archive section name and align section

Keep the section name consistent with the old
way of creating the CPIO archive. Also make sure the
section is word-aligned on all architectures by aligning
to an 8 byte boundary.

Revert "revert: Revert recent CPIO changes"

This will be fixed

This reverts commit 86a49071f7ca78038f5b1b96248c362eb8edff3b.

revert: Revert recent CPIO changes

These changes fix an EABI reqirement for newer compilers, but still lead
to invalid elfloader images for some platform configurations.

cpio: Fix new cpio file importing strategy

- Use CMAKE_C_FLAGS when compiling the .S into a .o to pick up the
correct arch settings
- Remove the duplicate _archive_start and _archive_end symbols from
riscv's elfloader linker script.

Fix cmake style in cpio.cmake

Now should pass the style checker.

Replace tabs with spaces in cpio.cmake

For some reson the tools don't like tabs. Bitbucket doesn't display
them properly for instance. Get rid of the one I'd introduced.

Fix breakage after EABI fix patch

The 'a' flag on the archive section got lost, meaning the whole
content of the built CPIO blob was put into a section that was
discarded on load. It turns out that the Sabre (at least) also needs
write access to the CPIO blob.

Also, IA32 and X86_64 use the same compiler; use the -m32 flag to
force 32-bit ELF output for IA32.

Fix ARM EABI mismatch with recent compilers

Recent (gcc-8 and above) compilers generate ARM EABI 5 files;
change the way we generate included binary blobs so they too are EABI 5.

cmake-tool, cpio: Run --reproducible check once

The pattern in CMake is to run a command and cache the result so that
the command doesn't need to be run every time the build files are
reprocessed. We test for the existence of the cpio --reproducible flag
that suppresses meta data changes that prevent reproducible builds.
Additionally there will be an info message printed on build
initialisation if the flag isn't available.

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.

trivial: style this repo

cmake: fix shared 'temp' folder when MakeCPIO is used multiple times

when MakeCPIO is used more than once in a project with parallel building,
temps files for different cpio archives are stored in the same temp folder,
which could make 'rmdir temp' fail since other building processes' temp files
are still there. Simply extending the temp folder's name to temp_${output_name}
could resolve this issue.

cpio.cmake: Refactor if block to be more readable

cpio.cmake: Reset commands variable

Not resetting it before use can lead to random commands in the task.

cmake: Move cpio functions to own file