jemalloc: Move generated files into lib/libc tree
Make it easire to update jemalloc by moving the FreeBSD specific files
and the generated files into lib/libc. This allows us to regenerate them
more easily, and emphasizes a bit that we may have to regenerate stuff
from upstream.
This is necessary to also unthin the import from the vendor branch as
well (which will be needed to simplify the imports in the future since
we are trying to use contrib/jemalloc for two different things).
No functional change.
Sponsored by: Netflix