malloc from jl_gc_managed_malloc is not counted in the heap size.

[qinsoon]

The bytes allocated by jl_gc_managed_malloc() in gc-common.c is counted in gc_num.malloc and gc_num.allocd, but it is not counted into MMTk heap.

https://github.com/mmtk/julia/blob/861f151c7fbec1d8431a804795bd3d3598b77e3b/src/gc-common.c#L760

We use JULIA_MALLOC_BYTES to count malloc bytes in functions like jl_gc_counted_malloc (in mmtk-gc.c, https://github.com/mmtk/julia/blob/861f151c7fbec1d8431a804795bd3d3598b77e3b/src/mmtk-gc.c#L380).