Fix encoding/decoding of string literals for big-endian systems

Fix encoding/decoding of string literals for big-endian systems

Per SPIR-V spec, a string literal's UTF-8 octets are encoded packed into
words with little-endian convention. Explicitly perform that encoding
instead of assuming that the host system is little-endian.

Note that this change requires corresponding fixes in SPIRV-Tools.

Fixes #202