posix: device_io: provide stdin, stdout, stderr variables

Create a compatibility Kconfig option for libc's that provide
their own stdin, stdout, stderr variables.

These are POSIX variables, so eventually we may want to
consider defaulting this option to 'y' and maybe providing
some zvfs abstraction for them.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>