Add ".dword" directive

Summary:
The ".dword" directive is a synonym for ".xword" and is used used
by klibc, a minimalistic libc subset for initramfs.

Reviewers: t.p.northover, nickdesaulniers

Reviewed By: nickdesaulniers

Subscribers: nickdesaulniers, javed.absar, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61719

llvm-svn: 360381