Feature request: Add crate feature to support jemalloc's --without-export

[itamarst]

Use case: I am LD_PRELOADing a library that is (partially) written in Rust, and will likely be using jemalloc for that code.

The more symbols are public, the more chances of things going wrong. As such, I would like to not have any of the _rjem_* symbols public, but just used internally by the rust code. I believe this is what --without-export does.

(I can probably workaround this with ld version scripts or something, but since jemalloc already has this option...)