Fix tospace/newspace confusion.

Fix tospace/newspace confusion.

This did not manifest in my testing on NetBSD because it happened
that on NetBSD, the tospace and newspace are always separated by more
than 4 GB, so the bogus jmprel32_offset was never used during GC,
e.g.:

   newspace=0x17dd7f8 tospace=0x7f7feeb00000 delta=17523427132673

I suspect it is the cause of Chris's problems on macOS, so this
should fix Chris's issue on macOS and speed things up even more for
everyone.