fix: Change all static variables to thread when fuzzing

[nathaniel-brough]

Describe the PR
Currently the AFL fuzzing engine makes the assumption that it can run a library in multiple threads without any shared state. To make this assumption consistent with tinyusb we need to make static globals thread local when fuzzing.

Additional context
Discussed here:
#1715 (comment)
Actual issue here (see stability)
https://oss-fuzz.com/fuzzer-stats?project=tinyusb&fuzzer=afl&job=afl_asan_tinyusb&group_by=by-fuzzer

[hathach]

thank you very much for your PR and sorry for the delay, I was too busy with other paid works. I back off a bit on using TU_STATIC, instead I would prefer to add _fuzz_thread as additional keyword. Even though it is a bit more verbose, the static keyword is more apparent to other user and easier to read.

I also rename macro FUZZ to _FUZZ to avoid if user application define it in the future .

Note: we could change _fuzz_thread to less verbose word in the future if you have any other suggestion. For now, I think this is good for merge

[HiFiPhile]

Personally I prefer to use TU_STATIC instead of adding _fuzz_thread keyword.

For most people who doesn't use fuzzing it's confusing what this keyword does, and I feel also the code is less neat putting a test keyword inside the stack.

For anyone who contribute they also need to learn what fuzzer does instead of copy TU_STATIC from existing code.

[hathach]

_fuzz_thread is __thread which is an actual keyword for thread_local (https://gcc.gnu.org/onlinedocs/gcc/Thread-Local.html#Thread-Local) . I was thinking to actual make use of it later on. Though you made the point, I think we should stick with tu_static instead.