checksyscalls: move instance functionality into generic code

checksyscalls: move instance functionality into generic code

On MIPS the checksyscalls.sh script may be executed multiple times.
Currently these multiple executions are executed on each build as kbuild
see that the commands have changed each time.

Use a dedicated stamp file for each different invocation to avoid the
spurious executions.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://patch.msgid.link/20260402-kbuild-missing-syscalls-v3-3-6641be1de2db@weissschuh.net
Signed-off-by: Nicolas Schier <nsc@kernel.org>