ia64: Always define IA64_USE_NEW_STUB as a flag macro

And keep the previous definition if it exists.  This allows
disabling IA64_USE_NEW_STUB while keeping USE_DL_SYSINFO defined.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
(cherry picked from commit 18bd9c3d)