gdb: fix "gdb.error: No struct named reflect.rtype."

With a trivial Golang-built program loaded in gdb-7.8.90.20150214-7.fc23.x86_64
I get this error:

(gdb) source ./src/runtime/runtime-gdb.py
Loading Go Runtime support.
Traceback (most recent call last):
  File "./src/runtime/runtime-gdb.py", line 230, in <module>
    _rctp_type = gdb.lookup_type("struct reflect.rtype").pointer()
gdb.error: No struct type named reflect.rtype.
(gdb) q

No matter if this struct should or should not be in every Golang-built binary
this change should fix that with no disadvantages.

Change-Id: I0c490d3c9bbe93c65a2183b41bfbdc0c0f405bd1
Reviewed-on: https://go-review.googlesource.com/5521Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/runtime-gdb.py

@@ -227,8 +227,6 @@ def lookup_type(name):
 	except gdb.error:
 		pass
 
-_rctp_type = gdb.lookup_type("struct reflect.rtype").pointer()
-
 
 def iface_commontype(obj):
 	if is_iface(obj):
@@ -238,7 +236,7 @@ def iface_commontype(obj):
 	else:
 		return
 
-	return go_type_ptr.cast(_rctp_type).dereference()
+	return go_type_ptr.cast(gdb.lookup_type("struct reflect.rtype").pointer()).dereference()
 
 
 def iface_dtype(obj):