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.
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