Fix "psb CSYNC" and "bti C".

Fix "psb CSYNC" and "bti C".

psb CYSNC was not finding that CSYNC was a correct spelling.
The problem was upper case version was being put in the
wrong hashtable.  This fixes the problem by using the
correct hashtable.
Also adds testcases for the upper case versions.

        * config/tc-aarch64.c (md_begin): Use correct
        hash table for uppercase version of hint.
        * testsuite/gas/aarch64/system-2.s: Extend psb case to uppercase.
        * testsuite/gas/aarch64/system-2.d: Update.

Change-Id: If43f8b85cacd24840d596c3092b0345e5f212766