arch: randomize initial user stack pointer

Inside the page randomization of initial user stack pointer is not only a part
of ASLR implementation but also a performance improvement that helps
eliminating aligned 64 kB data access.

Minimal user stack size is increased to 8 kB in order to ensure that regardless
of initial stack pointer value there is still enough space on stack.

headers/private/system/thread_defs.h

@@ -15,7 +15,7 @@
 #define USER_STACK_GUARD_SIZE		(4 * B_PAGE_SIZE)		// 16 kB
 #define USER_MAIN_THREAD_STACK_SIZE	(16 * 1024 * 1024)		// 16 MB
 #define USER_STACK_SIZE				(256 * 1024)			// 256 kB
-#define MIN_USER_STACK_SIZE			(4 * 1024)				// 4 KB
+#define MIN_USER_STACK_SIZE			(8 * 1024)				// 8 kB
 #define MAX_USER_STACK_SIZE			(16 * 1024 * 1024)		// 16 MB