ZJIT: Add --zjit-mem-size

[k0kubun]

It's hard-coded to use 64MiB for the code region, but this should be configurable. For --zjit-mem-size, you need to implement zjit_alloc_size() in stats.rs.

--zjit-exec-mem-size might be still useful for testing purposes, but I think we should hide it from --help and the documentation even if we add the option.