core/ipc_helper: Allow popping all signed value types with RequestParser

There's no real reason this shouldn't be allowed, given some values sent
via a request can be signed. This also makes it less annoying to work
with popping enum values, given an enum class with no type specifier
will work out of the box now.

It's also kind of an oversight to allow popping s64 values, but nothing
else.

src/core/hle/ipc_helpers.h

@@ -362,6 +362,11 @@ inline u32 RequestParser::Pop() {
     return cmdbuf[index++];
 }
 
+template <>
+inline s32 RequestParser::Pop() {
+    return static_cast<s32>(Pop<u32>());
+}
+
 template <typename T>
 void RequestParser::PopRaw(T& value) {
     std::memcpy(&value, cmdbuf + index, sizeof(T));
@@ -392,6 +397,16 @@ inline u64 RequestParser::Pop() {
     return msw << 32 | lsw;
 }
 
+template <>
+inline s8 RequestParser::Pop() {
+    return static_cast<s8>(Pop<u8>());
+}
+
+template <>
+inline s16 RequestParser::Pop() {
+    return static_cast<s16>(Pop<u16>());
+}
+
 template <>
 inline s64 RequestParser::Pop() {
     return static_cast<s64>(Pop<u64>());