Update gcc to 8.X

We should build Tor Browser using gcc 7.3.0. Firefox meta is https://bugzilla.mozilla.org/show_bug.cgi?id=1320654. The only thing to check is https://hg.mozilla.org/mozilla-central/rev/48ff50f0db50. Porting guide is https://gcc.gnu.org/gcc-7/porting_to.html. LRA is the default https://gcc.gnu.org/wiki/LRAIsDefault. -mindirect-branch=thunk is available.