LibC: Make scanf always copy its va_list

On x86_64 GCC implements va_list as an array. This makes the syntax
for taking a pointer to it break & crash. The workaround / solution is
to create a copy. Since va_list is a tiny struct referencing the
actual varargs, this is little overhead (especially compared to
va_args itself)

Userland/Libraries/LibC/scanf.cpp

@@ -383,6 +383,9 @@ extern "C" int vsscanf(const char* input, const char* format, va_list ap)
 
  int elements_matched = 0;
 
+ va_list copy;
+ __builtin_va_copy(copy, ap);
+
  while (!format_lexer.is_eof()) {
  format_lexer.ignore_while(isspace);
  if (!format_lexer.next_is('%')) {
@@ -537,7 +540,7 @@ extern "C" int vsscanf(const char* input, const char* format, va_list ap)
  }
  }
 
- auto* ap_or_null = !suppress_assignment ? (va_list*)&ap : nullptr;
+ auto* ap_or_null = !suppress_assignment ? (va_list*)&copy : nullptr;
 
  // Now try to read.
  switch (conversion_specifier) {
@@ -616,6 +619,7 @@ extern "C" int vsscanf(const char* input, const char* format, va_list ap)
  }
  }
  }
+ va_end(copy);
 
  return elements_matched;
 }