AArch64: remove over-zealous use of CHECK-NEXT

It turns out some platforms (e.g. Windows) lay out their llvm-mc slightly
differently with extra newlines; there was no real reason for the test lines to
be consecutive, so this relaxes the FileCheck.
AArch64: use full triple for ELF tests

These tests rely specifically on the names of ELF relocations, let alone any
other detail. There's no way they'd work if LLVM was emitting something else by
default.