Implement allocation fast path for Immix

[peterzhu2118]

This commit implements a fast path that inlines mmtk_post_alloc for Immix. The benchmark results show a decent speed up in allocation performance.

GC.disable

i = 0
while i < 10_000_000
  Object.new
  i += 1
end

Before:

Time (mean ± σ):     506.3 ms ±   5.0 ms    [User: 442.8 ms, System: 108.8 ms]
Range (min … max):   497.8 ms … 513.2 ms    10 runs

After:

Time (mean ± σ):     473.9 ms ±   2.4 ms    [User: 409.4 ms, System: 108.8 ms]
Range (min … max):   470.4 ms … 478.1 ms    10 runs