GCC's default visibility is "public" but can be changed to
"hidden" with the argument "-fvisibility=hidden".
Tested with x86_64 / Ubuntu Linux 18.04 LTS:
[before]
$ nm -g -C libmimalloc.so | grep "T " | wc -l
142
[after]
$ nm -g -C libmimalloc.so | grep "T " | wc -l
93
This patch does not change the build on macOS.