alpha: remove CONFIG_OSF4_COMPAT flag from syscall table

Remove CONFIG_OSF4_COMPAT config flag from system call
table - systbls.S and to keep the same feature, add the
flag in osf_sys.c.

One of the patch in this patch series will generate the
system call table file. In order to come up with a common
implementation across all architecture, we need this change.

Signed-off-by: Firoz Khan <firoz.khan@linaro.org>
Signed-off-by: Matt Turner <mattst88@gmail.com>