jemalloc: Import version 5.2.1

Create a new vendor branch for jemalloc, using the following for future
reference:

% git worktree add --orphan -b vendor/jemalloc ../vendors/jemalloc
% cd ../vendors/jemalloc
% git checkout -b vendor/jemalloc

% cd ../../upstreams
% git clone git@github.com:jemalloc/jemalloc.git
% cd jemalloc
% git checkout 5.2.1 # hash ea6b3e973b477b8061e0076bb257dbd7f3faa756

% cp -r * ../../vendors/jemalloc
% cd ../../vendors/jemalloc
% autoconf
% ./configure --enable-autoconf --with-version=5.2.1-0-gea6b3e973b477b8061e0076bb257dbd7f3faa756
% git add *
% git commit -m<this message>
% git tag -a vendor/jemalloc/5.2.1
% git push --follow --set-upstream freebsd vendor/jemalloc

Note: This purposely doesn't thin the vendor branch. That will be done
in the next step to create the merge commit that boostraps future
imports.

Sponsored by: Netflix