This commit moves the GC from free list allocation to
bit mark allocation. Instead of using the bitmaps
generated during the mark phases to generate free
list and then using the free lists for allocation we
allocate directly from the bitmaps.

The change in the garbage benchmark

name              old time/op  new time/op  delta
XBenchGarbage-12  2.22ms ± 1%  2.13ms ± 1%  -3.90%  (p=0.000 n=18+18)

Change-Id: I17f57233336f0ca5ef5404c3be4ecb443ab622aa