spidermonkey_52: 52.2.1gnome1 → 52.6.0

Update to latest upstream version and remove some flags that caused errors:

--enable-threadsafe was removed in https://bugzilla.mozilla.org/show_bug.cgi?id=1253466, since SpiderMonkey is thread-safe by default now https://bugzilla.mozilla.org/show_bug.cgi?id=1031529.

--enable-gcgenerational also disappeared for some reason.