Add ability to target platforms to individually include user-mode
support for system calls from "stat" group of system calls.

This change is related to new nanoMIPS platform in the sense that
it supports a different set of "stat" system calls than any other
target. nanoMIPS does not support structures stat and stat64 at
all. Also, support for certain number of other system calls is
dropped in nanoMIPS (those are most of the time obsoleted system
calls).

Without this patch, build for nanoMIPS would fail.
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Reviewed-by: NAleksandar Markovic <amarkovic@wavecomp.com>
Signed-off-by: NAleksandar Markovic <amarkovic@wavecomp.com>
Signed-off-by: NStefan Markovic <smarkovic@wavecomp.com>