linux-user: redirect openat calls

linux-user: redirect openat calls

While Mikhail fixed /proc/self/maps, it was noticed openat calls are
not redirected currently. Some archs don't have open at all, so
openat needs to be redirected.

Fix this by consolidating open/openat code to do_openat - open
is implemented using openat(AT_FDCWD, ... ), which according
to open(2) man page is identical.

Since all targets now have openat, remove the ifdef around sys_openat
and openat: case in do_syscall.

Cc: Mikhail Ilin <m.ilin@samsung.com>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>