1.0.5.43: change memory layout on FreeBSD

1.0.5.43: change memory layout on FreeBSD

* Move read-only-space and neighbours to lower space to avoid
  potential conflicts with runtime user space.

* Move dynamic-space to higher space so that kern.maxdsiz can be
  increased at least 1GB.

Brief memory layout of FreeBSD:

+------------+ -- 0x00000000
|    free    |
+------------+ -- 0x08000000
| user space |
+------------+ -- brk (grows downward)
~            ~
+------------+ -- 0x28000000 (0x08000000 + maxdsiz)
| mmap,shlib |
~            ~
+----------- + -- stack top (grows upward)
| stack space|
+------------+ -- 0xC0000000
|   kernel   |
+------------+ -- 0xFFFFFFFF