Allow multiple occurrences of the frames-invalid annotation in gdb.cp/annota2.exp

Allow multiple occurrences of the frames-invalid annotation in gdb.cp/annota2.exp

Hi,
I see one fail on aarch64-linux testing,

  FAIL: gdb.cp/annota2.exp: watch triggered on a.x (timeout)

because GDB prints two frames-invalid annotation but the test expects
only one.

next^M
^M
^Z^Zpost-prompt^M
^M
^Z^Zstarting^M
^M
^Z^Zframes-invalid^M
^M
^Z^Zframes-invalid^M
^M
Note I also see the fail on Debian-s390x-m64 too.
https://sourceware.org/ml/gdb-testers/2015-q4/msg07291.html

The test shouldn't only expect one frames-invalid annotation, because
there can be multiple times of stop/resume before the user visible
stop.  Ulrich did something similar before
https://www.sourceware.org/ml/gdb-patches/2009-06/msg00118.html

This patch only changes ${frames_invalid} to \(${frames_invalid}\)*
in the regexp pattern.

The patch below fixes the fail on aarch64-linux.

gdb/testsuite:

2015-11-27  Yao Qi  <yao.qi@linaro.org>

	* gdb.cp/annota2.exp: Allow multiple occurrences of the
	frames-invalid annotation.