Introduce CONFIG_DEFAULT_TASK_STACKSIZE

* The appropriate size of stack varies among archs.
  E.g. for 64-bit sim, 2048 is way too small, especially when the task
  happens to use host OS functionalities.
  I plan to allow an arch provide its own default.

* I plan to use this to replace hardcoded "STACKSIZE = 2048" in APPDIR.

libs/libc/unistd/Kconfig

@@ -105,7 +105,7 @@ config POSIX_SPAWN_PROXY_STACKSIZE
 
 config TASK_SPAWN_DEFAULT_STACKSIZE
 	int "Default task_spawn Stack Size"
-	default 2048
+	default DEFAULT_TASK_STACKSIZE
 	depends on !ARCH_ADDRENV
 	---help---
 		The actual size to use for the child task's stack can be set with

libs/libc/wqueue/Kconfig

@@ -23,7 +23,7 @@ config LIB_USRWORKPRIORITY
 
 config LIB_USRWORKSTACKSIZE
 	int "User mode worker thread stack size"
-	default 2048
+	default DEFAULT_TASK_STACKSIZE
 	---help---
 		The stack size allocated for the lower priority worker thread.  Default: 2K.