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/LibCoredump/Backtrace.cpp

@@ -50,6 +50,9 @@ Backtrace::Backtrace(Reader const& coredump, const ELF::Core::ThreadInfo& thread
 #elif ARCH(AARCH64)
     auto start_bp = m_thread_info.regs.x[29];
     auto start_ip = m_thread_info.regs.pc;
+#elif ARCH(RISCV64)
+    auto start_bp = m_thread_info.regs.x[7];
+    auto start_ip = m_thread_info.regs.pc;
 #else
 #    error Unknown architecture
 #endif