[SCons] Refactor targets, symbols, optimizations.

Now matches Godot `master` target names and supports the same flags with
the following notable exceptions:
- The default target is "template_debug", since it's compatible with
  editor builds (and TOOLS_ENABLED is never used internally).
- separate_debug_symbols is still not supported, and will be done in a
  separate commit.

test/SConstruct

@@ -25,9 +25,7 @@ if env["platform"] == "macos":
     )
 else:
     library = env.SharedLibrary(
-        "demo/bin/libgdexample.{}.{}.{}{}".format(
-            env["platform"], env["target"], env["arch_suffix"], env["SHLIBSUFFIX"]
-        ),
+        "demo/bin/libgdexample{}{}".format(env["suffix"], env["SHLIBSUFFIX"]),
         source=sources,
     )