[gdb] Fix 'not in executable format' error message

[gdb] Fix 'not in executable format' error message

With trying to load a non-executable file into gdb, we run into PR26880:
...
$ gdb -q -batch test.c
"0x7ffc87bfc8d0s": not in executable format: \
  file format not recognized
...

The problem is caused by using %ps in combination with the error function
(note that confusingly, it does work in combination with the warning
function).

Fix this by using plain "%s" instead.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-08-23  Tom de Vries  <tdevries@suse.de>

	PR gdb/26880
	* gdb/exec.c (exec_file_attach): Use %s instead of %ps in call to
	error function.

gdb/testsuite/ChangeLog:

2021-08-23  Tom de Vries  <tdevries@suse.de>

	PR gdb/26880
	* gdb.base/non-executable.exp: New file.