Fix attach-pie-noexec.exp fail on native-extended-gdbserver

Fix attach-pie-noexec.exp fail on native-extended-gdbserver

When I examine the buildbot fails, I see this fail on
native-extended-gdbserver,

 Attaching to process 13529^M
 "target:/scratch/yao/gdb/build-git/x86_64/gdb/testsuite/gdb.base/attach-pie-noexec (deleted)": could not open as an executable file: No such file or directory^M
 (gdb) FAIL: gdb.base/attach-pie-noexec.exp: attach

if I run tests with board file unix, it doesn't exist,

 Attaching to process 13869^M
 /scratch/yao/gdb/build-git/x86_64/gdb/testsuite/gdb.base/attach-pie-noexec (deleted): No such file or directory.^M
 (gdb) PASS: gdb.base/attach-pie-noexec.exp: attach

the test expects to see the period at the end of the error message,

  gdb_test "attach $testpid" "Attaching to process $testpid\r\n.*: No such file or directory\\." "attach"

however the period is missing when running with native-extended-gdbserver.

in exec.c:exec_file_attach, GDB has two places may throw errors [1] and [2],

      if (load_via_target)
	{
	  ...
	}
      else
	{
 	  ...
	  if (scratch_chan < 0)
	    perror_with_name (filename);  <--- [1]
	}

	...
      if (!exec_bfd)
	{
	  error (_("\"%s\": could not open as an executable file: %s"), <-- [2]
		 scratch_pathname, bfd_errmsg (bfd_get_error ()));
	}

perror_with_name [1] append a period at the end of error message,
but error [2] doesn't.  This fix is to add a period at the end of the
error message.  Note that this fail is shown up on 7.9 release as well.

gdb:

2015-07-13  Yao Qi  <yao.qi@linaro.org>

	* exec.c (exec_file_attach): Add period at the end of error
	message.