Expect optional "arch=" when executing "-stack-list-frames" on gdb.arch/amd64-invalid...

Expect optional "arch=" when executing "-stack-list-frames" on gdb.arch/amd64-invalid-stack-top.exp

Another case of incomplete regexp.  The problem is very similar to the
one happening with gdb.arch/amd64-invalid-stack-middle.exp.

The output when GDB is compiled with "--enable-targets" is:

  (gdb) interpreter-exec mi "-stack-list-frames"
  ^done,stack=[frame={level="0",addr="0x00000000004005e7",func="func2",arch="i386:x86-64"}]

While the output when "--enable-targets" is not specified is:

  (gdb) interpreter-exec mi "-stack-list-frames"
  ^done,stack=[frame={level="0",addr="0x00000000004005e7",func="func2"}]

The fix is, again, to extend the current regexp and expect for the
optional "arch=" part.  With this patch, the test now passes on both
scenarios.

OK?

gdb/testsuite/ChangeLog:
2018-09-18  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.arch/amd64-invalid-stack-top.exp: Expect optional
	"arch=" keyword when executing "-stack-list-frames".