[Base] Add operator<< string conversion for vec128_t

This allows `catch` to print out the contents of a particular vector when diagnosing how a `REQUIRE` expression has failed.

src/xenia/base/vec128.h

@@ -257,6 +257,8 @@ static inline vec128_t vec128b(uint8_t x0, uint8_t x1, uint8_t x2, uint8_t x3,
 
 std::string to_string(const vec128_t& value);
 
+std::ostream& operator<<(std::ostream& os, const vec128_t& value);
+
 }  // namespace xe
 
 #endif  // XENIA_BASE_VEC128_H_