[SIMD.js] Implement SIMD128 data load and store APIs

It allows developers to load or store 1, 2, 3, and 4 elements of SIMD128
data from or to variable typed arrays.

The APIs inlucde:
SIMD.float32x4.load
SIMD.float32x4.loadX
SIMD.float32x4.loadXY
SIMD.float32x4.loadXYZ
SIMD.float32x4.store
SIMD.float32x4.storeX
SIMD.float32x4.storeXY
SIMD.float32x4.storeXYZ
SIMD.float64x2.load
SIMD.float64x2.loadX
SIMD.float64x2.store
SIMD.float64x2.storeX
SIMD.int32x4.load
SIMD.int32x4.loadX
SIMD.int32x4.loadXY
SIMD.int32x4.loadXYZ
SIMD.int32x4.store
SIMD.int32x4.storeX
SIMD.int32x4.storeXY
SIMD.int32x4.storeXYZ

This commit only optimizes accessing float32x4 in Float32Array and Int8Array,
accessing int32x4 in Int32Array and Int8Array as well as accessing float64x2
in Float64Array and Int8Array.

TODO=optimize for x64
TEST=mjsunit/simd/loadstore
BUG=XWALK-2865