rustdoc: unmarked code fences are doctests too
mk: Build jemalloc with -ffunction-sections

It's quite possible that small programs don't use all of jemalloc, and building
with -ffunction-sections and -fdata-sections allows the linker (via
--gc-sections) to strip out all unused code at link time. This decreases the
size of a "hello world" executable for me from 716K to 482K with no measurable
impact on link time. After this patch jemalloc is still the largest portion of
our hello world executables, but this helps cut down on the size at least
somewhat!
auto merge of #20183 : japaric/rust/doctests, r=alexcrichton

#20075 introduced a bug where unmarked code fences weren't considered as doctests. This PR fixes the logic.

---

This passed `check-stage1-rustdoc`, and I manually checked that:

``` rust
//! ```
//! println!("Hello")
//! ```
//!
//! ``` rust
//! println!("Hello")
//! ```
//!
//! ``` sh
//! println!("Hello")
//! ```
//!
//! ``` ignore
//! println!("Hello")
//! ```
```

Generated:

``` rust
running 3 tests
test _2 ... ignored
test _0 ... ok
test _1 ... ok
```

I'd love to add that as a test, but I have no idea how to do that with our testing infrastructure. If anyone knows how, do let me know!

r? @alexcrichton 
@seanmonstar feedback?
auto merge of #20218 : alexcrichton/rust/jemalloc-sections, r=luqmana

It's quite possible that small programs don't use all of jemalloc, and building
with -ffunction-sections and -fdata-sections allows the linker (via
--gc-sections) to strip out all unused code at link time. This decreases the
size of a "hello world" executable for me from 716K to 482K with no measurable
impact on link time. After this patch jemalloc is still the largest portion of
our hello world executables, but this helps cut down on the size at least
somewhat!