[llvm-objcopy] Change Segment::Type from uint64_t to uint32_t

Summary:
In both Elf{32,64}_Phdr, the field Elf{32,64}_World p_type is uint32_t.

Also reorder the fields to be similar to Elf64_Phdr (which is different
from Elf32_Phdr but quite similar).

Reviewers: rupprecht, jhenderson, jakehehrlich, alexshap, espindola

Reviewed By: rupprecht

Subscribers: emaste, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D55618

llvm-svn: 348985