Core: Alter the kernel string functions to use std::string instead of const char*.

Most functions already operate on std::strings. This also removes the need to manually null terminate thread names.

src/core/hle/service/gsp.h

@@ -137,7 +137,7 @@ public:
      * Gets the string port name used by CTROS for the service
      * @return Port name of service
      */
-    const char *GetPortName() const {
+    std::string GetPortName() const {
         return "gsp::Gpu";
     }