[CVS ci] invalidate method cache in store_global

[Leopold Toetsch]

If a name is stored into a namespace that starts with two underscores,
the method cache is invalidated now.

The follwing program runs now as expected:

.sub main @MAIN
.local pmc o, cl
newclass cl, "Foo"
subclass cl, cl, "Bar"
$I0 = typeof cl
o = new $I0
print o
$P0 = global "ok2"
store_global "Bar", "__get_string", $P0
print o
.end
.sub ok2
.return("ok 2\n")
.end
.namespace [ "Foo" ]
.sub __get_string
.return("ok 1\n")
.end

leo