[llvm-readobj] Only allow 4-byte pr_data

Summary: AMD64 psABI says: "The pr_data field of each property contains a 4-byte unsigned integer." Thus we don't need to handle 8-byte pr_data.

Reviewers: mike.dvoretsky, grimar, craig.topper, xiangzhangllvm, hjl.tools

Reviewed By: grimar

Subscribers: rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58103

llvm-svn: 353815