Fix warning_level=everything with GCC 8

This option was introduced with GCC 8.1.0 as in the original commit, but the
value wasn't right initially and was volatile during the 8 series.

To avoid this, this commit moves the warning to 9.1.0 (the next version we
generally care about), since we don't want to get too deep into the weeds of
point releases, and a warning not being used yet in some particular version of
GCC isn't a big deal.

mesonbuild/compilers/mixins/gnu.py

@@ -196,11 +196,13 @@ gnu_common_warning_args = {
         "-Wduplicated-branches",
     ],
     "8.1.0": [
-        "-Wattribute-alias=2",
         "-Wcast-align=strict",
         "-Wsuggest-attribute=cold",
         "-Wsuggest-attribute=malloc",
     ],
+    "9.1.0": [
+        "-Wattribute-alias=2",
+    ],
     "10.1.0": [
         "-Wanalyzer-too-complex",
         "-Warith-conversion",