[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.