[gdb/testssuite] Skip aarch64 in skip_gdbserver_test if no xml support

[gdb/testssuite] Skip aarch64 in skip_gdbserver_test if no xml support

On aarch64-linux, with a gdb build without libexpat, so without xml support, I
run into:
...
(gdb) builtin_spawn attach-no-multi-process^M
attach 26808^M
Attaching to Remote target^M
warning: Can not parse XML target description; XML support was disabled at \
  compile time^M
Reading symbols from attach-no-multi-process...^M
Remote 'g' packet reply is too long (expected 788 bytes, got 796 bytes): ... ^M
...

The test-case checks for skip_gdbserver_tests and that one contains a check
for xml support:
...
    # If GDB is lack of XML support, and targets, like arm, have
    # multiple target descriptions, GDB doesn't know which target
    # description GDBserver uses, and may fail to parse 'g' packet
    # after connection.
    if { [gdb_skip_xml_test]
	 && ([istarget "arm*-*-linux*"]
	      || [istarget "mips*-*-linux*"]
	      || [istarget "powerpc*-*-linux*"]
	      || [istarget "s390*-*-linux*"]
	      || [istarget "x86_64-*-linux*"]
	      || [istarget "i\[34567\]86-*-linux*"]) } {
	return 1
    }
...
but it doesn't trigger because aarch64 is missing.

Fix this by adding istarget "aarch64*-*-linux*".

Tested on aarch64-linux.

Approved-By: Luis Machado <luis.machado@arm.com>