Userland+Tests: Add initial riscv64 support

Author: https://github.com/spholz
Commit: 6824d2a788
Pull-request: https://github.com/SerenityOS/serenity/pull/21139
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/Hendiadyoin1
Reviewed-by: https://github.com/kleinesfilmroellchen ✅

Userland/Libraries/LibDebug/DebugSession.cpp

@@ -377,6 +377,8 @@ FlatPtr DebugSession::single_step()
     regs.rflags |= TRAP_FLAG;
 #elif ARCH(AARCH64)
     TODO_AARCH64();
+#elif ARCH(RISCV64)
+    TODO_RISCV64();
 #else
 #    error Unknown architecture
 #endif
@@ -394,6 +396,8 @@ FlatPtr DebugSession::single_step()
     regs.rflags &= ~(TRAP_FLAG);
 #elif ARCH(AARCH64)
     TODO_AARCH64();
+#elif ARCH(RISCV64)
+    TODO_RISCV64();
 #else
 #    error Unknown architecture
 #endif