Add spec test support for V128. (#1110)
* Add spec test support for V128.
* This algorithm, however simple, produces the digits in reverse order. Remember to reverse them again before printing.
* Correct order of the quads of the v128.
Given (v128.const i32x4 0x01020304 0x05060708 0x090a0b0c 0x0d0e0f00), the wabt v128 struct will contain 0x01020304 in v128_bits.v[0]. If a wasm program uses v128.store to store this constant then loads each byte with i32.load8_u, the wabt interpreter produces the order 04 03 02 01, 08 07 06 05, 0c 0b 0a 09, 00 0f 0e 0d.
* Add a parser for v128 and implement support in spectest-interp.
* Move WriteUint128 to literal.cc, add tests.
Rewrite so as to not use string streams. Roll repeated code into a loop.
Rename functions to comply with style guide, use anonymous namespace.
* Now that v128 has operator==, we can use it here.
* In a spectest with a binary module, use the same features for that module as for the outer spectest.
