[llvm-readobj] Dump GNU_PROPERTY_X86_FEATURE_2_{NEEDED,USED} notes in .note.gnu.property

Summary: And change the output ("X86 features" -> "x86 feature") a bit.

Reviewers: grimar, xiangzhangllvm, hjl.tools, rupprecht

Reviewed By: rupprecht

Subscribers: rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58112

llvm-svn: 353908