Fix big endian serialization

Fix big endian serialization

Big endian serialization was broken because:
- it partially relied on `WORDS_ENDIAN` (unconditionally undef'd in cutils.h)
- endianness was not handled at all in the bc reader.

Modifications:
- remove `WORDS_ENDIAN`
- use `bc_put_u32()` / `bc_put_u64()` in `JS_WriteBigInt()`
- use `bc_get_u32()` / `bc_get_u64()` in `JS_ReadBigInt()`
- handle host endianness in `bc_get_u16()`, `bc_get_u32()`, `bc_get_u64()` and
  `JS_ReadFunctionBytecode()`

- handle optional littleEndian argument as specified in
  `js_dataview_getValue()` and `js_dataview_setValue()`