[HTML5] Use absolute path for JS lib/pre/externs.

Ensure better compatibility when emcc which may run some tools from
different paths (e.g. closure compiler).

This fixes externs include issues with modern emcc using the closure
compiler.

(cherry picked from commit eaedc92c61abf2958529833f640ef26d219d2274)

platform/javascript/SCsub

@@ -27,11 +27,11 @@ if env["javascript_eval"]:
     sys_env.AddJSLibraries(["js/libs/library_godot_javascript_singleton.js"])
 
 for lib in sys_env["JS_LIBS"]:
-    sys_env.Append(LINKFLAGS=["--js-library", lib])
+    sys_env.Append(LINKFLAGS=["--js-library", lib.abspath])
 for js in env["JS_PRE"]:
-    sys_env.Append(LINKFLAGS=["--pre-js", env.File(js).path])
+    sys_env.Append(LINKFLAGS=["--pre-js", js.abspath])
 for ext in env["JS_EXTERNS"]:
-    sys_env["ENV"]["EMCC_CLOSURE_ARGS"] += " --externs " + ext.path
+    sys_env["ENV"]["EMCC_CLOSURE_ARGS"] += " --externs " + ext.abspath
 
 build = []
 if env["gdnative_enabled"]: