rtos: rtx5: ARMCC5: move the variables in .bss.os section to ZI section

rtos: rtx5: ARMCC5: move the variables in .bss.os section to ZI section

The thread stacks and other large variables in rtx_lib.c were directed
to .bss.os section, which puts them to zero initialized section in GCC,
but on ARMC5 they were put to data section which consumes precious ROM.

In reality the data section compression may make this a NOP change,
but it will at least make the total result statistics now more reliable.

Effects of this PR on the output of mbed compile of one application:
--8<--8<--
> --- memory_before_bss_change.txt	2018-06-18 16:45:51.928844313 +0300
> +++ memory_after_bss_change.txt	2018-06-18 16:57:27.753532437 +0300
> @@ -5,6 +5,9 @@
>  +----------------------------------------------------------+--------+-------+-------+
>  | Module                                                   |  .text | .data |  .bss |
>  +----------------------------------------------------------+--------+-------+-------+
> @@ -366,7 +369,7 @@
>  | mbed-os/rtos/TARGET_CORTEX/rtx5/RTX/Source/rtx_kernel.o  |    801 |   164 |     0 |
> -| mbed-os/rtos/TARGET_CORTEX/rtx5/RTX/Source/rtx_lib.o     |    406 |  2117 |     0 |
> +| mbed-os/rtos/TARGET_CORTEX/rtx5/RTX/Source/rtx_lib.o     |    406 |     1 |  2116 |
>  | mbed-os/rtos/TARGET_CORTEX/rtx5/RTX/Source/rtx_memory.o  |    260 |     0 |     0 |
> @@ -412,9 +415,9 @@
> -| Subtotals                                                | 163559 |  3380 | 15168 |
> +| Subtotals                                                | 163559 |  1264 | 17284 |
>  +----------------------------------------------------------+--------+-------+-------+
>  Total Static RAM memory (data + bss): 18548 bytes
> -Total Flash memory (text + data): 166939 bytes
> +Total Flash memory (text + data): 164823 bytes