Docs: update va_arg example with valid x86_64 va_list type.

Docs: update va_arg example with valid x86_64 va_list type.

The given example was overflowing its alloca and segfaulting if actually run on
x86, so it's a good idea to provide something that works there too.

Patch by Ramkumar Ramachandra.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221077 91177308-0d34-0410-b5e6-96231b3b80d8