Update the stdsimd submodule

[alexcrichton]

This brings in a few updates:

• Update wasm intrinsic naming for atomics
• Update and reimplement most simd128 wasm intrinsics
• Other misc improvements here and there, including a small start to
  AVX-512 intrinsics

[rust-highfive]

r? @TimNN

(rust_highfive has picked a reviewer for you, use r? to override)

[rust-highfive]

⚠️ Warning ⚠️

• These commits modify submodules.

[TimNN]

R=me if travis is green

[alexcrichton]

@bors: r=TimNN

[bors]

📌 Commit 7d2c6509258c6d8280eb0bf6bd9d27717f560d0f has been approved by TimNN

[alexcrichton]

@bors: rollup

[pietroalbini]

@bors r-

Failed in the rollup:

[00:03:04] error[E0441]: unrecognized platform-specific intrinsic function: `simd_select_bitmask`
[00:03:04]   --> src/libcore/../stdsimd/coresimd/simd_llvm.rs:52:5
[00:03:04]    |
[00:03:04] 52 |     pub fn simd_select_bitmask<M, T>(m: M, a: T, b: T) -> T;
[00:03:04] 
[00:03:04] error: aborting due to previous error

[alexcrichton]

@bors: rollup- r=TimNN

Did some investigation locally and looks like the rollup may have gotten mixed up and forgot the last commit of stdsimd, which is needed to fix that failure (and is oddly enough present in this PR...)

[bors]

📌 Commit 7d2c6509258c6d8280eb0bf6bd9d27717f560d0f has been approved by TimNN

[pietroalbini]

@bors retry

Trying to wake bors up.

[pietroalbini]

@bors are you still there?

[bors]

🍰 Activated

[pietroalbini]

@bors r=TimNN

[bors]

📌 Commit 5e5823c has been approved by TimNN

[bors]

⌛ Testing commit 5e5823c with merge e57ab26...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: TimNN
Pushing e57ab26 to master...