Switch the build process to using Rust's new build system called
rustbuild. At the same time stop using jemalloc on all platforms and
switch to the system allocator since that was simplier to do.
(cherry picked from commit a4fe235317)
We shouldn't need to patch rustc to find the target but instead pass the
full path to the target spec. This should solve #138.
(cherry picked from commit 06a138a4a4)