Add AT_RSEQ_* from Linux 6.3 to elf.h

Linux 6.3 adds constants AT_RSEQ_FEATURE_SIZE and AT_RSEQ_ALIGN; add
them to glibc's elf.h.  (Recall that, although elf.h is a
system-independent header, so far we've put AT_* constants there even
if Linux-specific, as discussed in bug 15794.  So rather than making
any attempt to fix that issue, the new constants are just added there
alongside the existing ones.)

Tested for x86_64.