checksyscalls: ignore fstat to silence build warning on LoongArch

fstat is replaced by statx on the new architecture, so an exception is
added to the checksyscalls script to silence the following build warning
on LoongArch:

  CALL    scripts/checksyscalls.sh
<stdin>:569:2: warning: #warning syscall fstat not implemented [-Wcpp]

Link: https://lkml.kernel.org/r/1678175940-20872-1-git-send-email-yangtiezhu@loongson.cn


Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Suggested-by: WANG Xuerui <kernel@xen0n.name>
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>