build: [gn] centralize GN args under build/args/*.gn

Devs can still locally override or add settings by adding lines after
the `import` statement in args.gn.

This will help make sure that everyone has a common set of build args,
and will allow us to update everyone's build args in git.

build/args/debug.gn

@@ -0,0 +1,4 @@
+import("all.gn")
+is_debug = true
+is_component_build = true
+use_jumbo_build = true