error: use __INITIAL_SP from cmsis instead of RTX one

We used to require INITIAL_SP as rtx target headers define it. This should not be required, as
cmsis already defines symbol  __INITIAL_SP for all toolchains.

Fixes ARMmbed#14432

Author: 0xc0170

platform/source/mbed_error.c

@@ -29,6 +29,7 @@
 #include "platform/internal/mbed_error_hist.h"
 #include "drivers/MbedCRC.h"
 #include "mbed_rtx.h"
+#include "cmsis_compiler.h"
 #ifdef MBED_CONF_RTOS_PRESENT
 #include "rtx_os.h"
 #endif
@@ -518,7 +519,7 @@ static void print_stack_dump(uint32_t stack_start, uint32_t stack_size, uint32_t
             // PSP mode. Then SP_reg is more correct.
             psp_sp = mfc->SP_reg;
         }
-        uint32_t msp_size = MAX(0, (int)INITIAL_SP - (int)msp_sp);
+        uint32_t msp_size = MAX(0, (int)__INITIAL_SP - (int)msp_sp);
         print_stack_dump_core(msp_sp, msp_size, msp_sp, "MSP");
 
         stack_sp = psp_sp;