Relax type-printer regexp in libstdc++ test suite

The libstdc++ test suite checks whether gdb type printers are
available like so:

    set do_whatis_tests [gdb_batch_check "python print(gdb.type_printers)" \
			   "\\\[\\\]"]

This regexp assumes that the list of printers is empty.  However,
sometimes it's convenient to ship a gdb that comes with some default
printers, causing this to erroneously report that gdb is "too old".

I believe the intent of this check is to ensure that gdb.type_printers
exists -- not to check its starting value.  This patch changes the
check to accept any Python list as output.

Note that the patch doesn't look for the trailing "]".  I tried this
but in my case the output was too long for expect.  It seemed fine to
just check the start, as the point really is to reject the case where
the command prints an error message.

libstdc++-v3/ChangeLog

	* testsuite/lib/gdb-test.exp (gdb-test): Relax type-printer
	regexp.

libstdc++-v3/testsuite/lib/gdb-test.exp

@@ -107,8 +107,12 @@ proc gdb-test { marker {selector {}} {load_xmethods 0} } {
 	}
     }
 
+    # A very old version of gdb will not have the type_printers
+    # global.  Some organizations may ship a gdb that has some default
+    # type printers, so accept any list output as indication that the
+    # global exists.
     set do_whatis_tests [gdb_batch_check "python print(gdb.type_printers)" \
-			   "\\\[\\\]"]
+			   "\\\[.+"]
     if {!$do_whatis_tests} {
 	send_log "skipping 'whatis' tests - gdb too old"
     }