open64: ensure that openat check works

For ARC builds atleast, __NR_xxx was not trickling into in open64.c
causing open64->openat a needless detour via open

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>