arm: SYSCALL_REGISTER == SYSCALL_N

Set TRACY_SYSCALL_N to the proper register; any call to
tracy_modify_syscall_args would fail and cause segmentation faults in the child
processes.
tracy: Add detach functionality.

TRACY_HOOK_DETACH_CHILD is provided for detaching from syscall hooks
tracy_detach_child is provided for more control

tests/attach-detach: Attach to a process, and detach as soon as it performs a
write() system call.
tracy: Fix compilation & clean tracy_debug_current
trac.h: Remove unused macro.
tests: fuzz-write: Mess with write()'s to stdout