semihosting: Create qemu_semihosting_guestfd_init

semihosting: Create qemu_semihosting_guestfd_init

For arm-compat, initialize console_{in,out}_gf;
otherwise, initialize stdio file descriptors.

This will go some way to cleaning up arm-compat, and
will allow other semihosting to use normal stdio.

Reviewed-by: Luc Michel <lmichel@kalray.eu>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>