linker: Reduce number of mmap()/prctl() calls in block allocator

Given that the block allocator does not free allocated memory unless
all objects are freed, we can allocate a large chunk of memory at a
time.  This gives us:
  1. Higher space efficiency, especially when the objects being
     allocated has a large size.
  2. Much less mmap()/prctl() calls.
  3. Much less vm_area_struct objects.
While this increases the size of virtual memory allocated, the number
of dirty pages would be unchanged or less.

Test result on a Go device:
  - Zygote starts 24ms faster on average.
  - Kernel vm_area_struct usage 138KB less, across the entire system.

Bug: 112073665
Test: Boot and check the result from 'showmap'.
Change-Id: Iece631a5081151dd1e5bf25dbddac9b264bcdcff