Fix endianness bug.

Looks like llvm-readobj is the only customer of this code, and apparently
there's no test to cover this function. I'll write it after finishing
plumbing from llvm-objdump to there.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202915 91177308-0d34-0410-b5e6-96231b3b80d8