fixup_mmaps_after_fork errors building ruby 3.4.1 with high make -j

[Jeremy Drake]

I've been trying to track down why ruby 3.4.1 is failing to build for
msys2.  I've now confirmed that this same issue reproduces with
upstream Cygwin 3.5.4 and 3.6.0-0.335.gb879cd1661ad, so I thought I'd
bring it up here:

when building ruby with a high -j value (I'm doing make -j21), I get
errors of the sort:
     0 [main] miniruby 9272 fhandler_dev_zero::fixup_mmap_after_fork:
requested 0x6FFFE6A60000 != 0x0 mem alloc base 0x0, state 0x10000, size
17545331867648, Win32 error 1455
   2717 [main] miniruby 9272
D:\S\ruby\src\build-x86_64-pc-msys\miniruby.exe: *** fatal error in forked
process - recreate_mmaps_after_fork_failed
      0 [main] miniruby 9241 dofork: child -1 - forked process 24744 died
unexpectedly, retry 0, exit code 0x100, errno 11

I did not see this happen with ruby 3.3.7, or when building with -j1.
That address 0x6FFFE6A60000 seems to reoccur.  The size seems completely
off the wall.

Any ideas on how to properly debug this would be appreciated.