mk: Allow usage of a local jemalloc install

This adds a new configure option, --jemalloc-root, which will specify a location
at which libjemalloc_pic.a must live. This library is then used for the build
triple as the jemalloc library to link.
travis: Use a pre-installed jemalloc
mk: Allow using a locally compiled libuv.a

Closes #5563
auto merge of #14792 : alexcrichton/rust/local-jemalloc, r=brson

This configures the makefiles to copy a local jemalloc/libuv library into place instead of building the local copy of one. Additionally, this switches our travis builds to using the system-provided jemalloc instead of a custom-built jemalloc to exercise this functionality.