aarch64: testsuite: move sysreg tests into sysreg sub-directory

This patch moves the existing sysreg tests for AArch64 into a subdirectory
(sysreg). The number of test files related to system registers grew
relatively big with time and makes the browsing of those files difficult.
Moreover, the difference of naming for the failure, working, and
feature-specific scenarios causes the tests not to appear next to one
another in the exploration tree when it is ordered alphabetically.

gas/testsuite/gas/aarch64/sysreg/aarch64-sysreg.exp

@@ -0,0 +1,23 @@
+# Copyright (C) 2024 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston, MA 02110-1301, USA.
+
+#
+# Some AArch64 tests for system registers
+#
+
+if {[istarget aarch64*-*-*]} {
+    run_dump_tests [lsort [glob -nocomplain $srcdir/$subdir/*.d]]
+}