mbed_boot_arm_std.c: remove redundant compiler check

Check is not needed since this file is exclusive for ARM compiler.
It causes problems on ARMC6.

Author: mprse

rtos/TARGET_CORTEX/TOOLCHAIN_ARM_STD/mbed_boot_arm_std.c

@@ -28,13 +28,11 @@ __value_in_regs struct __argc_argv __rt_lib_init(unsigned heapbase, unsigned hea
 void _platform_post_stackheap_init(void);
 
 #if !defined(ISR_STACK_SIZE)
-#if (defined(__CC_ARM))
 extern uint32_t               Image$$ARM_LIB_STACK$$ZI$$Base[];
 extern uint32_t               Image$$ARM_LIB_STACK$$ZI$$Length[];
 #define ISR_STACK_START       ((unsigned char*)Image$$ARM_LIB_STACK$$ZI$$Base)
 #define ISR_STACK_SIZE        ((uint32_t)Image$$ARM_LIB_STACK$$ZI$$Length)
 #endif
-#endif
 
 #if !defined(HEAP_START)
 /* Defined by linker script */