Shenandoah: allocate collection set bitmap at lower addresses

LRB midpath has the in-cset check that tests the address against cset bitmap. The default address for that bitmap is quite huge, so we end up emitting a large address immediate for it:

  ...65: mov %rax,%r10
  ...68: shr $0x17,%r10
  ...6c: movabs $0x7f3c5533d000,%r11
  ...76: cmpb $0x0,(%r11,%r10,1)
  ...7b: je 0x00007f3c408f4e55

We can help it if we put cset bitmap at lower addresses.

For example with:
  https://cr.openjdk.java.net/~shade/8245464/webrev.01/

  ...65: mov %rax,%r10
  ...68: shr $0x17,%r10
  ...6c: mov $0x10000,%r11d
  ...72: cmpb $0x0,(%r11,%r10,1)
  ...77: je 0x00007f868c8ef355

Immediate used to take 10 bytes, now it takes 6 bytes.