bsp/gdbarmsim: Change syscall functions to not clash with RTEMS functions.

The syscall functions overlapped with RTEMS, for example _write, _read, etc.
Change these to be internal to the BSP and avoid any clash with names in
RTEMS. Add support for SWI_Write0.

Change the console driver to use SWI_Write0. This outputs the character
to the host's stdout. Writing to file name 0 is not captured and managed
by GDB's simulation code while the SWI_Write0 is. The managed stdout
data is encapulated in the MI protocol while writes to file handle 0 are
dropped by GDB when in MI mode.