Memory leak with low latency GC

[junjihashimoto]

Running static-mnist-cnn with low latency gc leaks memory.
I've checked the leak on the current master branch (2021/06/04 b98c8f0) .
Running mallocTrim after gc has no effect.
The presence or absence of mallocTrim and the leak are irrelevant.

Running static-mnist-cnn without low latency gc does not leak memory.