Use reserved bytes in OSThread structs for wut related thread specifics

[Maschell]

Open for discussion.

Recently __wut_getreent got implemented which uses OSGetThreadSpecific/OSSetThreadSpecific to store some thread specific data. This works fine for regular homebrew, but for WUPS/WUMS (Plugin/Module system) we might end up using a reent within a (Cafe OS) thread we do not control (e.g. due to function replacing/patching). In this case using/overriding the "regular" thread specifics might then result in undefined behaviour. To avoid this I propose to the use the "reserved" bytes of the OSThread struct for custom thread specifics which are exclusively reserved for wut.

Contra:

• Using the reserved bytes is/feels quite hacky. Forcing every homebrew do this this feels even more hacky.
• If Nintendo drops an update and decides to use this reserved bytes all homebrew break

Pro:

• wut homebrew can again use all "regular" thread specifcs
• Using these reserved bytes is documented in wut. If this won't get merged I probably end up using the bytes anyway for WUPS/WUMS, but then it's less obvious they are actually used by something.
• We won't need separate getreent implementations for WUPS/WUMS.

• I am personally not really sure how to fix it. Using these reserved bytes feels indeed very hacky but seems overall to be the easiest and cleanest solution.

[Maschell]

closed in favour of #324

[fincs]

Superseded by #324. The "hacky" bits will not be part of wut, instead they will be user provided through overrides.