Support -vmheap_size > 2G

Today, simply due to using the same static bitmap size for both vmcode and vmheap, we limit -vmheap_size to 2G. We would like to allow increasing its size, to avoid things like #2115, to allow using guard pages with larger page sizes (xref #4415).

Xref #2115: beyond-vmheap slowdowns allocating memory Xref #3570 on -vm_size 2G Xref #4415 on parceling out sub-page pieces