LibELF: Support weak symbols when using BIND_NOW

When using BIND_NOW (e.g. via -Wl,-z,now) we would fail to load ELF
images while doing relocations when we encounter a weak symbol. Instead
we should just patch the PLT entry with a null pointer.

This can be reproduced with:

$ cat test.cpp
int main()
{
    std::cout << "Hello World!" << std::endl;
}
$ g++ -o test -Wl,-z,now test.cpp
$ ./test
did not find symbol while doing relocations for library test: _ITM_RU1

Userland/Libraries/LibELF/DynamicObject.cpp

@@ -454,13 +454,15 @@ VirtualAddress DynamicObject::patch_plt_entry(u32 relocation_offset)
     auto symbol = relocation.symbol();
     u8* relocation_address = relocation.address().as_ptr();
 
+    VirtualAddress symbol_location;
     auto result = DynamicLoader::lookup_symbol(symbol);
-    if (!result.has_value()) {
+    if (result.has_value()) {
+        symbol_location = result.value().address;
+    } else if (symbol.bind() != STB_WEAK) {
         dbgln("did not find symbol while doing relocations for library {}: {}", m_filename, symbol.name());
         VERIFY_NOT_REACHED();
     }
 
-    auto symbol_location = result.value().address;
     dbgln_if(DYNAMIC_LOAD_DEBUG, "DynamicLoader: Jump slot relocation: putting {} ({}) into PLT at {}", symbol.name(), symbol_location, (void*)relocation_address);
 
     *(FlatPtr*)relocation_address = symbol_location.get();