* sysdeps/unix/sysv/linux/ldsodefs.h (HAVE_AUX_XID): Define if
__ASSUME_AT_XID is defined.
* sysdeps/unix/sysv/linux/kernel-features.h (__ASSUME_AT_XID): Define
for 2.4.1 and up.
* sysdeps/generic/dl-sysdep.c (_dl_sysdep_start): Optimize handling
of AT_UID, AT_EUID, AT_GID, and AT_EGID.