These can occur in the following scenario:

- thread 1 has blocked signal A
- thread 2 has not blocked signal A
- signal A is sent to a process with thread 1 and thread 2
- thread 1 is simultaneously unblocking signal A, and thus jumps to the
  trampoline
- thread 2 is awoken, but thread 1 won the "fetch_andn" to clear the
  signal bit.

If the signal asm is jumped to automatically after each timer interrupt,
this scenario will also be relatively common due to that.

Implement syscall restart

See merge request redox-os/relibc!483

Fix missing modulo in random_bool()

See merge request !482

Refactor rand48 functions with less unsafe

See merge request redox-os/relibc!473

Add instructions to test relibc against a recipe

See merge request redox-os/relibc!471