[gdb/testsuite] Fix gdb.base/maint.exp on powerpc64le

[gdb/testsuite] Fix gdb.base/maint.exp on powerpc64le

On powerpc64le-linux, I ran into:
...
FAIL: gdb.base/maint.exp: maint print objfiles: symtabs
...

The problem is that:
- the "Cooked index in use" line occurs twice in the gdb output:
  - once for exec maint, and
  - once for "Object file system-supplied DSO".
- the matching of the second "Cooked index in use" also consumes
  the "Symtabs:" string, and consequently the corresponding
  clause does not trigger and $symtabs remains 0.

Fix this by limiting the output of the command to the exec.

Tested on x86_64-linux and powerpcle-linux.