[boehm] Avoid a ENOMEM when allocating across an unallocated page

[boehm] Avoid a ENOMEM when allocating across an unallocated page

A very rare allocation pattern could cause the boehm free list to call
the GC_unmap_gap function, which actually calls down into munmap().
Darwins virtual memory manager will return a KERN_INVALID_ADDRESS
which is translated into a ENOMEM, if mprotect attempts to change
the protection of a range which includes an unallocated page.
We address this by just mmap() back to ANON, instead of actually
unallocating the page.