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

SPIRV/SPVRemapper.cpp

@@ -297,15 +297,21 @@ namespace spv {
     std::string spirvbin_t::literalString(unsigned word) const
     {
         std::string literal;
+        const spirword_t * pos = spv.data() + word;
 
         literal.reserve(16);
 
-        const char* bytes = reinterpret_cast<const char*>(spv.data() + word);
-
-        while (bytes && *bytes)
-            literal += *bytes++;
-
-        return literal;
+        do {
+            spirword_t word = *pos;
+            for (int i = 0; i < 4; i++) {
+                char c = word & 0xff;
+                if (c == '\0')
+                    return literal;
+                literal += c;
+                word >>= 8;
+            }
+            pos++;
+        } while (true);
     }
 
     void spirvbin_t::applyMap()