[llvm-dlltool] Mention arm64 in the lists of architecture alternatives
[llvm-dlltool] Mention arm64 in the lists of architecture alternatives

This was missed in SVN r310223 when arm64 support was added.

Differential Revision: https://reviews.llvm.org/D37588

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@312776 91177308-0d34-0410-b5e6-96231b3b80d8

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