Userland+Tests: Add initial riscv64 support

Userland/Libraries/LibELF/Arch/riscv64/entry.S

@@ -1 +1,11 @@
-# Intentionally empty.
+/*
+ * Copyright (c) 2023, Sönke Holz <sholz8530@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+.globl _invoke_entry
+.hidden _invoke_entry
+.type _invoke_entry,@function
+_invoke_entry: # (argc, argv, envp, entry)
+    jr a3

Userland/Libraries/LibELF/Arch/riscv64/plt_trampoline.S

@@ -1 +1,14 @@
-# Intentionally empty.
+/*
+ * Copyright (c) 2023, Sönke Holz <sholz8530@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+.p2align 4
+.globl _plt_trampoline
+.hidden _plt_trampoline
+.type _plt_trampoline,@function
+
+_plt_trampoline:
+    // FIXME: Implement the PLT trampoline.
+    unimp

Userland/Libraries/LibELF/DynamicLinker.cpp

@@ -723,8 +723,12 @@ void ELF::DynamicLinker::linker_main(DeprecatedString&& main_program_path, int m
     if (s_do_breakpoint_trap_before_entry) {
 #if ARCH(AARCH64)
         asm("brk #0");
-#else
+#elif ARCH(RISCV64)
+        asm("ebreak");
+#elif ARCH(X86_64)
         asm("int3");
+#else
+#    error "Unknown architecture"
 #endif
     }