Revert partially "Remove Cortex A support from CMSIS/RTOS"

This reverts commit 5f29445103 - but only partially.

cmain.S for IAR is still required to keep mbed SDK boot process as it was. This
startup file for IAR is active for non-rtos projects.

Fixes #4330

cmsis/TOOLCHAIN_IAR/cmain.S

@@ -0,0 +1,81 @@
+/**************************************************
+ *
+ * Part two of the system initialization code, contains C-level
+ * initialization, thumb-2 only variant.
+ *
+ * Copyright 2006 IAR Systems. All rights reserved.
+ *
+ * $Revision: 59783 $
+ *
+ **************************************************/
+
+
+; --------------------------------------------------
+; Module ?cmain, C-level initialization.
+;
+
+
+        SECTION SHT$$PREINIT_ARRAY:CONST:NOROOT(2)
+        SECTION SHT$$INIT_ARRAY:CONST:NOROOT(2)
+
+        SECTION .text:CODE:NOROOT(2)
+
+        PUBLIC  __cmain
+        ;; Keep ?main for legacy reasons, it is accessed in countless instances of cstartup.s around the world...
+        PUBLIC  ?main
+        EXTWEAK __iar_data_init3
+        EXTWEAK __iar_argc_argv
+        EXTERN  __low_level_init
+        EXTERN  __call_ctors
+        EXTERN  main
+        EXTERN  exit
+        EXTERN  __iar_dynamic_initialization
+        EXTERN mbed_sdk_init
+        EXTERN SystemInit
+        
+        THUMB
+__cmain:
+?main:
+
+; Initialize segments.
+; __segment_init and __low_level_init are assumed to use the same
+; instruction set and to be reachable by BL from the ICODE segment
+; (it is safest to link them in segment ICODE).
+
+          FUNCALL __cmain, __low_level_init
+        bl      __low_level_init
+        cmp     r0,#0
+        beq     ?l1
+          FUNCALL __cmain, __iar_data_init3
+        bl      __iar_data_init3
+        MOVS    r0,#0             ;  No parameters
+        FUNCALL __cmain, mbed_sdk_init
+        BL      mbed_sdk_init
+        MOVS    r0,#0             ;  No parameters        
+          FUNCALL __cmain, __iar_dynamic_initialization
+        BL      __iar_dynamic_initialization   ; C++ dynamic initialization  
+
+?l1:
+        REQUIRE ?l3
+
+        SECTION .text:CODE:NOROOT(2)
+
+        PUBLIC  _main
+        PUBLIC _call_main
+        THUMB
+
+__iar_init$$done:                 ; Copy initialization is done
+
+?l3:
+_call_main:
+        MOVS    r0,#0             ;  No parameters  
+          FUNCALL __cmain, __iar_argc_argv
+        BL      __iar_argc_argv   ; Maybe setup command line
+
+          FUNCALL __cmain, main
+        BL      main
+_main:
+          FUNCALL __cmain, exit
+        BL      exit
+
+        END