This little PR fixes the systhreads assertion failure from #12712.

By good old printf-debugging:tm: I found out that the failure happens
on a boundary condition when 4/5 minor heap pointers align:

 young_start: 4029677568
 young_limit: 4029677568
 young_trigger: 4029677568
 young_ptr: 4029677568
 young_end: 4029685760

Multiple reruns confirmed that the condition is the same (the
addresses naturally vary) every time the assertion fails.

Here's my current understanding:

• The minor heap uses a young_ptr bump allocator starting
  from young_end and working towards young_start.
• young_trigger typically marks the mid-point between the two,
  and hitting it triggers a major gc slice.
• Other domains may set young_limit to UINTNAT_MAX to notify
  a domain (irrelevant in the present systhread test with only 1 domain)

With the above in mind, AFAICS, the above condition may happen when

• a previous young_ptr allocation just fit perfectly
• young_trigger is set to young_start by caml_poll_gc_work

This also explains why running with a reduced minor heap
OCAMLRUNPARAM="s=1024" helps to reproduce the failure.

The PR simply proposes to adjust the assertion to account for the
boundary condition.

With the fix, the systhread test from #12712 runs fine across 20 iterations.
To confirm it further, I've also locally run

• the ocaml compiler test suite
• the multicoretests test suite both with and without the debug runtime