Expose SIMD.Float32x4 type to Javascript. This CL exposes the constructor function, defines type related information, and implements value type semantics. It also refactors test/mjsunit/samevalue.js to test SameValue and SameValueZero (issue 4124).

Fix runtime-atomics for Win 10 SDK and remove volatile (Chromium issues 440500, 491424).

Performance and stability improvements on all platforms.

Cr-Commit-Position: refs/heads/4.6.10@{#1}
Cr-Branched-From: 124d2011ea1dfc3d21e9d7f639cdd69ccc0fbeb2-refs/heads/master@{#29691}