This change adds mimalloc as a git submodule so that `make` doesn't
try to clone mimalloc repo. This change should allow building mold
with the nix package manager, which doesn't provide network access
while building.
This change may conflict with a local `mimalloc` directory. If you
already have checked out mold and `mimalloc` directory exists in the
mold directory, you may need to remove it by `rm -r mimalloc` before
running `make`.
Requested by David Yamnitsky.
Fixes https://github.com/rui314/mold/issues/50