posix: options: add xsi realtime option group

Add a POSIX Option Group called XSI_REALTIME (with Kconfig
option CONFIG_XSI_REALTIME).

When XSI_REALTIME is selected (or when required POSIX Options
are enabled), define _XOPEN_REALTIME to be something other
than -1 (_XOPEN_VERSION seemed appropriate).

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