rustbuild: Build jemalloc and libbacktrace only once (take 2)
This is a rebase of https://github.com/rust-lang/rust/pull/38583 without any additions, but with implemented @alexcrichton's suggestions.
~~This includes `exists(Makefile)` => `cfg(stage0)` suggestion... but it will break cross-compilation, no? Are `libstd/liballoc_jemalloc` cross-compiled for `target != host` built during `stage0`?~~
r? @alexcrichton
57ecd7aa4b