LibELF: Implement PLT relocations for x86_64

Userland/Libraries/LibELF/Arch/x86_64/plt_trampoline.S

@@ -8,5 +8,42 @@
     .globl _plt_trampoline
     .hidden _plt_trampoline
     .type _plt_trampoline,@function
-_plt_trampoline:
+_plt_trampoline: # (object, relocation_index)
-    int3
+     # save flags/registers (https://stackoverflow.com/questions/18024672/what-registers-are-preserved-through-a-linux-x86-64-function-call)
+    pushfq
+    pushq %rax
+    pushq %rcx
+    pushq %rdx
+    pushq %rsi
+    pushq %rdi
+    pushq %r8
+    pushq %r9
+    pushq %r10
+    pushq %r11
+
+    movq 80(%rsp), %rdi # object
+    movq 88(%rsp), %rsi # relocation_index
+
+    # offset = index * sizeof(Elf64_Rela)
+    shlq $3, %rsi
+    leaq (%rsi, %rsi, 2), %rsi
+
+    call _fixup_plt_entry@PLT
+
+    movq %rax, 88(%rsp) # replace object argument with symbol address
+
+     # restore flags/registers
+    popq %r11
+    popq %r10
+    popq %r9
+    popq %r8
+    popq %rdi
+    popq %rsi
+    popq %rdx
+    popq %rcx
+    popq %rax
+    popfq
+
+    addq $8, %rsp # remove relocation_index argument
+
+    retq

Userland/Libraries/LibELF/DynamicLoader.cpp

@@ -506,7 +506,7 @@ DynamicLoader::RelocationResult DynamicLoader::do_relocation(const ELF::DynamicO
             u8* relocation_address = relocation.address().as_ptr();
 
             if (m_elf_image.is_dynamic())
-                *(u32*)relocation_address += (FlatPtr)m_dynamic_object->base_address().as_ptr();
+                *(FlatPtr*)relocation_address += (FlatPtr)m_dynamic_object->base_address().as_ptr();
         }
         break;
     }

Userland/Libraries/LibELF/DynamicObject.cpp

@@ -449,7 +449,11 @@ NonnullRefPtr<DynamicObject> DynamicObject::create(const String& filename, Virtu
 VirtualAddress DynamicObject::patch_plt_entry(u32 relocation_offset)
 {
     auto relocation = plt_relocation_section().relocation_at_offset(relocation_offset);
+#if ARCH(I386)
     VERIFY(relocation.type() == R_386_JMP_SLOT);
+#else
+    VERIFY(relocation.type() == R_X86_64_JUMP_SLOT);
+#endif
     auto symbol = relocation.symbol();
     u8* relocation_address = relocation.address().as_ptr();