ARM64: accept ELF-relocated load/store insts without a #.

E.g. we print "ldr x0, [x0, :lo12:symbol]" so we need to accept that syntax
too.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207639 91177308-0d34-0410-b5e6-96231b3b80d8