examples/cmsis-rtos-api: Grow stacks for printing

The CMSIS RTOS API example needs additional stack to be able to use
serial_printf() with the standard C library (non-MicroLib). Allocate
threads with enough stack for printing.

Versions of the HAL since 8957b9cd5077 switch from using the small
MicroLib C library to the standard C library. As such, our stack usage
per thread goes up for any threads that print, as serial_printf() uses
snprintf() from the C library. The standard C library's implementation
of snprintf() uses more stack than the MicroLib C library.

Signed-off-by: Jaeden Amero <jaeden.amero@arm.com>