[llvm-readelf] Allow prefix flags for -p and -x

Summary: This allows syntax like `llvm-readelf -p.data1 -x.data2`.

Reviewers: jhenderson

Reviewed By: jhenderson

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59965

llvm-svn: 357270