Update System Call entry point for IAR/ARM_CM4_MPU

Earlier the System Call entry from an unprivileged task looked like:
1. SVC for entering system call.
2. System call implementation.
3. SVC for exiting system call.

Now, the entry point needs to make only one system call and everything
else is handled internally.

Signed-off-by: Gaurav Aggarwal <[email protected]>

Author: aggarg