Increase interrupt stack size for K64F

The config store tests use more than 2K of the interrupt stack, causing
an overflow.  This patch bumps the K64F interrupt stack size to 4K
for ARMCC and GCC. The IAR interrupt stack is left untouched since
it is 32K.

rtos/rtx/TARGET_CORTEX_M/RTX_CM_lib.h

@@ -426,6 +426,9 @@ extern uint32_t __StackTop[];
 #else
 #define INITIAL_SP            (0x20030000UL)
 #endif
+#if defined(__CC_ARM) || defined(__GNUC__)
+#define ISR_STACK_SIZE        (0x1000)
+#endif
 
 #elif defined(TARGET_K22F)
 #define INITIAL_SP            (0x20010000UL)