[Patch AArch64] Warn on unpredictable stlxrb , stlxrh and stlxr cases.
    
    A recent case in golang highlighted that gas wasn't warning on these
    unpredictable cases in the architecture. Fixed thusly.
    
    I need to audit gcc to make sure we have early clobbers on the
    patterns but that's a separate patch.
    
    Tested aarch64-none-elf and gas
    
    Ok ?
    
    Ramana
    
    2018-06-29 Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>
    
            * config/tc-aarch64.c (warn_unpredictable_ldst): Add
            unpredictable cases for ldxp, stlxrb, stlxrh, stlxr.  *
            testsuite/gas/aarch64/diagnostic.s: New tests.  *
            testsuite/gas/aarch64/diagnostic.l: Adjust.