[mips] update expected output in test/ELF/mips-plt-r6.s

r306095 in LLVM has changed register positions for instruction aui.
This change updates the test that checks for aui.


git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@306173 91177308-0d34-0410-b5e6-96231b3b80d8