x32/cet: Support shadow stack during startup for Linux 6.10

Use RXX_LP in RTLD_START_ENABLE_X86_FEATURES.  Support shadow stack during
startup for Linux 6.10:

commit 2883f01ec37dd8668e7222dfdb5980c86fdfe277
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Fri Mar 15 07:04:33 2024 -0700

    x86/shstk: Enable shadow stacks for x32

    1. Add shadow stack support to x32 signal.
    2. Use the 64-bit map_shadow_stack syscall for x32.
    3. Set up shadow stack for x32.

Add the map_shadow_stack system call to <fixup-asm-unistd.h> and regenerate
arch-syscall.h.  Tested on Intel Tiger Lake with CET enabled x32.  There
are no regressions with CET enabled x86-64.  There are no changes in CET
enabled x86-64 _dl_start_user.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>