jemalloc: bootstramp vendor/jemalloc branch

Using the just created vendor/jemalloc (details on its creation there),
create a subtree merge commit to boostrap (thanks emaste for the recipe
for another bootstrap):
% git merge -s subtree -Xsubtree=contrib/jemalloc -Xours vendor/jemalloc --allow-unrelated-histories

Now, cleanup the imported sources:
% cd contrib/jemalloc
% cat FREEBSD-Xlist | xargs git rm -rf
% git commit --amend
% git diff freebsd/main main # shows only FREEBSD-Xlist cleanups as diffs

No changes intended.

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