Allow rt_sigreturn in runsc gofer

rt_sigreturn is required for signal handling (e.g., SIGSEGV for nil-pointer
dereference). Before this, nil-pointer dereferences cause a syscall violation
instead of a panic.

PiperOrigin-RevId: 274028767

runsc/fsgofer/filter/config.go

@@ -177,6 +177,7 @@ var allowedSyscalls = seccomp.SyscallRules{
 	syscall.SYS_RENAMEAT:        {},
 	syscall.SYS_RESTART_SYSCALL: {},
 	syscall.SYS_RT_SIGPROCMASK:  {},
+	syscall.SYS_RT_SIGRETURN:    {},
 	syscall.SYS_SCHED_YIELD:     {},
 	syscall.SYS_SENDMSG: []seccomp.Rule{
 		// Used by fdchannel.Endpoint.SendFD().