[CMAKE] Improve the FindJemalloc module

Summary:
Prefer the PIC version when available, and pull the dependencies as
needed.

Test Plan:

cmake -GNinja .. -DUSE_JEMALLOC_EXPERIMENTAL=ON

On Debian, check in the cmake output that libdl is required.

ninja all check

Require D6038:

cmake -GNinja .. -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Linux64.cmake -DUSE_JEMALLOC_EXPERIMENTAL=ON

Check in the cmake output that libdl is NOT required.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6055