Use 64bit sizes for sh_entsize.
Elf64_Shdr uses 64bit sizes (Elf64_Xword) to represent the section entry size
(sh_entsize). The internal LLVM representation maintains this field as a 32bit
value, regardless if Elf32 or Elf64. This change ensures that enormous
entsizes (values larger than 2^32) can be represented internally.