Test fixes from rollup

Closes #14888 (Allow disabling jemalloc as the memory allocator)
Closes #14905 (rustc: Improve span for error about using a method as a field.)
Closes #14920 (Fix #14915)
Closes #14924 (Add a Syntastic plugin for Rust.)
Closes #14935 (debuginfo: Correctly handle indirectly recursive types)
Closes #14938 (Reexport `num_cpus` in `std::os`. Closes #14707)
Closes #14941 (std: Don't fail the task when a Future is dropped)
Closes #14942 (rustc: Don't mark type parameters as exported)
Closes #14943 (doc: Fix a link in the FAQ)
Closes #14944 (Update "use" to "uses" on ln186)
Closes #14949 (Update repo location)
Closes #14950 (fix typo in the libc crate)
Closes #14951 (Update Sublime Rust github link)
Closes #14953 (Fix --disable-rpath and tests)