gdb: support rseq auxvs

Linux kernel commit commit 317c8194e6ae ("rseq: Introduce feature size
and alignment ELF auxiliary vector entries") introduced two new auxvs:
AT_RSEQ_FEATURE_SIZE and AT_RSEQ_ALIGN.  Support them in GDB.  This
fixes auxv.exp on kernels >= v6.3.

Change-Id: I8966c4d5c73eb7b45de6d410a9b28a6628edad2e
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30540


Approved-By: Tom Tromey <tom@tromey.com>