cmain IAR: add mbed main

[0xc0170]

This fixes #4602 issue, mbed_main should be invoked right before the real main

Tested on nucleo l476rg, using mbed_a21 test

Sorry for the formatting, my terminal was not set properly :)
before this patch:

{{timeout;20}}
              {{host_test_name;default_auto}}
                                             {{description;Call function mbed_main before main}}
                {{test_id;MBED_A21}}
                                    {{start}}
                                             MBED: main() starts now!
{{failure}}
           {{end}}

after this patch:

call before main()
{{timeout;20}}
              {{host_test_name;default_auto}}
                                             {{description;Call function mbed_main before main}}
                {{test_id;MBED_A21}}
                                    {{start}}
                                             MBED: main() starts now!
{{success}}
           {{end}}

cc @jeromecoutant @c1728p9

[jeromecoutant]

Approved

[0xc0170]

/morph test

[mbed-bot]

Result: FAILURE

Your command has finished executing! Here's what you wrote!

/morph test

Output

mbed Build Number: 616

Test failed!

[studavekar]

/morph test

[mbed-bot]

Result: SUCCESS

Your command has finished executing! Here's what you wrote!

/morph test

Output

mbed Build Number: 619

All builds and test passed!