pass correct flags to jemalloc for cross-compiling
auto merge of #7007 : thestinger/rust/jemalloc, r=catamorphism

It's still broken because `jemalloc`'s `configure` script doesn't work when the build triple doesn't match the target triple due to missing a way to get the page size. I'm not entirely sure how to pass a hardcoded value to autoconf/configure.

This at least gets it a bit further along.