[flang] Disable arm64 gcc builds in CI until they can be fixed

Currently the arm64 gcc builds in CI are getting stuck,
so disable these to get CI working again until they're fixed.

Original-commit: flang-compiler/f18@49803e62ee
Reviewed-on: https://github.com/flang-compiler/f18/pull/829

flang/.drone.star

@@ -43,6 +43,6 @@ def main(ctx):
         clang("amd64"),
         clang("arm64"),
         gcc("amd64"),
-        gcc("arm64"),
+        #gcc("arm64"),
     ]