Use system memory allocator in instrumented binaries

[Shnatsel]

As we've found in https://github.com/rust-lang/rust/issues/36705, jemalloc has abysmal fork performance in some kernel configurations, notably on recent Ubuntu versions.

Even on good configurations system default allocator has about 20% better fork performance than jemalloc.

This can be done in documentation, by adding the 2 lines described in https://doc.rust-lang.org/nightly/book/custom-allocators.html to the instrumentation instructions.