Add phase2 types behind a runtime flag.

[billbudge]

var simdPhase2 = true; // define and test SIMD Phase2 types.

Moves definitions around in polyfill so all phase2 definitions are in one 'if'. Adds float64x2 and bool64x2 types and functions.

[ljharb]

Ooh, can we also add an additional runtime flag to enable/disable the unshimmable parts, like typeof and ==?

[littledan]

@ljharb the tests already have such a flag for ==.