Change stack size of main thread for RZ_A1H

We changed the stack size of main thread for RZ_A1H.
We changed "OS_MAINSTKSIZE" from 2048 to 4096.
Because stack shortage was found in the automatic test by the CI System .

Author: TomoYamanaka

rtos/rtx/TARGET_CORTEX_A/RTX_Conf_CA.c

@@ -74,7 +74,7 @@
 //   <i> Defines stack size for main thread.
 //   <i> Default: 200
 #ifndef OS_MAINSTKSIZE
- #define OS_MAINSTKSIZE 2048
+ #define OS_MAINSTKSIZE 4096
 #endif
 
 #ifndef __MBED_CMSIS_RTOS_CA9