[AArch64] Fix an incorrect PLT entry.

This patch is difficult to test in isolation, so a subsequent patch will test
further.

Patch by Daniel Stewart <stewartd@codeaurora.org>!
Phabricator Revision: http://reviews.llvm.org/D5377

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@218418 91177308-0d34-0410-b5e6-96231b3b80d8