arch: armv6-m: Refactor interrupt stack related code

Summary:
- Apply the same logic for armv7-m
- NOTE: stack pointer alignment is 4-byte

Impact:
- Affects armv6-m with interrupt stack enabled

Testing:
- Built with freedom-kl25z:nsh (CONFIG_ARCH_INTERRUPTSTACK=2048)
- Not tested but should work

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

arch/arm/src/armv6-m/arm_assert.c

@@ -247,7 +247,7 @@ static void up_dumpstate(void)
    * stack?
    */
 
-  if (sp <= istackbase && sp > istackbase - istacksize)
+  if (sp < istackbase && sp > istackbase - istacksize)
     {
       /* Yes.. dump the interrupt stack */
 

arch/arm/src/armv6-m/arm_exception.S

@@ -270,7 +270,7 @@ exception_common:
 	.bss
 	.global	g_intstackalloc
 	.global g_intstackbase
-	.align	4
+	.balign	4
 g_intstackalloc:
 	.skip	(CONFIG_ARCH_INTERRUPTSTACK & ~3)
 g_intstackbase: