SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.
As suggested here: https://dolp.in/pr7059#pullrequestreview-125401778
More descriptive than having a std::tuple of FS::Mode, and lets us
give names to known triplets of modes (like in ES). Functions that
only forward mode arguments are slightly less verbose now too.
Keeps them all next to each other and deduplicates a few constants,
notably the PPC UIDs. Apparently I forgot that I already added them
for SetupStreamKey.
It was off by about 8 years because it was actually the same as the GC epoch, however, the reason it worked all this time was because the default RTC counter bias of the Wii was not 0, but a value that is about 8 years in seconds. This broke custom RTC as a custom RTC of the gc epoch was underflowing b ecause the wii epoch was thought to be much later.
It's not common code that could be reused for, say, Citra;
it's absolutely specific to Wii emulation and only used by the Dolphin
core, so let's move it there.
Another reason for doing this is to avoid having Common depend on Core.
