Change Config to make fastmem_pointer of zero valid.

This changes Dynarmic::A32/A64::Config to store fastmem_pointer in
a std::optional<uintptr_t>, allowing the user to pass a zero base
address for the guest memory, which can be used to effectively
implement a shared address space between the host and the guest.

tests/A32/test_arm_instructions.cpp

@@ -540,7 +540,7 @@ TEST_CASE("arm: Memory access (fastmem)", "[arm][A32]") {
 
     A32FastmemTestEnv env{backing_memory};
     Dynarmic::A32::UserConfig config{&env};
-    config.fastmem_pointer = backing_memory;
+    config.fastmem_pointer = reinterpret_cast<uintptr_t>(backing_memory);
     config.recompile_on_fastmem_failure = false;
     config.processor_id = 0;