cmake: emu: Support QEMU semihosting console emulation

The QEMU '-semihosting' option enables the emulation of the semihosting
mechanism that can be used to interface the Zephyr RTOS to the host
operating system.

In order to support semihosting console output, the QEMU semihosting
feature is enabled and its console is connected to the console backend
chardev.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>