[CMAKE] Disable Jemalloc for the native OSX build

Summary:
The software is triggering a segfault when built against jemalloc on OSX
(not when cross compiling).
This seems to be an issue with Berkeley DB since the issue disappears
when built without the wallet (and the tests pass), but for safety I
prefer to disable it completely by default on native OSX.

Test Plan:
On OSX:

ninja all check-all

Check the software starts IBD without segfaulting:

./src/bitcoind

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7705