... on different platforms.

Official rustdoc of
[`usize::to_le_bytes`](https://doc.rust-lang.org/std/primitive.usize.html#method.to_le_bytes)
displays signature

```
pub fn to_ne_bytes(self) -> [u8; 8]
```

which might be misleading: this function returns 4 bytes on 32-bit
systems.

It was fixed somewhere between 1.28.0 and 1.31.1

closes rust-lang#50518

Pulls in a fix for ensuring that wasm targets have code in
compiler-builtins for `ldexp` which LLVM can generate references to.

to_xe_bytes for isize and usize returns an array of different size

... on different platforms.

Official rustdoc of
[`usize::to_le_bytes`](https://doc.rust-lang.org/std/primitive.usize.html#method.to_le_bytes)
displays signature

```
pub fn to_ne_bytes(self) -> [u8; 8]
```

which might be misleading: this function returns 4 bytes on 32-bit
systems.

With this commit applied rustdoc for `isize` and `usize` is this:
<img width="740" alt="2019-04-15_0020" src="https://user-images.githubusercontent.com/28969/56100765-9f69b380-5f14-11e9-974c-daa25edaa881.png">

std: Update compiler-builtins crate

Pulls in a fix for ensuring that wasm targets have code in
compiler-builtins for `ldexp` which LLVM can generate references to.

…-obk

Add test for rust-lang#59972

This PR adds a test for rust-lang#59972, which was fixed in rust-lang#59897.

Closes rust-lang#59972.

r? @eddyb

test for rust-lang#50518

It was fixed somewhere between 1.28.0 and 1.31.1

closes rust-lang#50518

r? @estebank

Where's the best place to add this test? I *think* we want "compile-pass" for this test (no need to run a binary, and not running saves us a millisecond of process creation) , but there's no compile-pass anymore.

Should this be UI test with empty stdout, stderr and zero return code?

Document + Cleanup lang_items.rs

Byproduct of work on rust-lang#60607.

r? @oli-obk