um: Add SECCOMP support detection and initialization

This detects seccomp support, sets the global using_seccomp variable and
initilizes the exec registers. The support is only enabled if the
seccomp= kernel parameter is set to either "on" or "auto". With "auto" a
fallback to ptrace mode will happen if initialization failed.

Signed-off-by: Benjamin Berg <benjamin@sipsolutions.net>
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20250602130052.545733-7-benjamin@sipsolutions.net


[extend help with Kconfig text from v2, use exit syscall instead of libc,
 remove unneeded mctx_offset assignment, disable on 32-bit for now]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>