mseal sysmap: add arch-support txt

Add Documentation/features/core/mseal_sys_mappings/arch-support.txt

N/A: the arch is 32bits only and mseal is not supported in 32 bits,
     therefore N/A (until mseal is available in 32 bits kernel).

[jeffxu@chromium.org: update to v3]
  Link: https://lkml.kernel.org/r/20250324151537.1106542-2-jeffxu@google.com
Link: https://lkml.kernel.org/r/20250321032627.4147562-2-jeffxu@google.com


Signed-off-by: Jeff Xu <jeffxu@chromium.org>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Eric Dumaze <edumazet@google.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: guoweikang <guoweikang.kernel@gmail.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Meghana Malladi <m-malladi@ti.com>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>