kvm: Fix sigsys handler error message.

And remove bluepillSignal var for amd64; it's unused.

PiperOrigin-RevId: 675344862

pkg/sentry/platform/kvm/bluepill_amd64.go

@@ -18,16 +18,10 @@
 package kvm
 
 import (
-	"golang.org/x/sys/unix"
 	"gvisor.dev/gvisor/pkg/ring0"
 	"gvisor.dev/gvisor/pkg/sentry/arch"
 )
 
-var (
-	// The action for bluepillSignal is changed by sigaction().
-	bluepillSignal = unix.SIGSEGV
-)
-
 func bluepillArchVCPU(context *arch.SignalContext64) *vCPU {
 	return vCPUPtr(uintptr(context.Rax))
 }

pkg/sentry/platform/kvm/machine.go

@@ -828,7 +828,7 @@ func seccompMmapRules(m *machine) {
 	seccompMmapRulesOnce.Do(func() {
 		// Install the handler.
 		if err := sighandling.ReplaceSignalHandler(unix.SIGSYS, addrOfSigsysHandler(), &savedSigsysHandler); err != nil {
-			panic(fmt.Sprintf("Unable to set handler for signal %d: %v", bluepillSignal, err))
+			panic(fmt.Sprintf("Unable to set handler for signal %d: %v", unix.SIGSYS, err))
 		}
 		rules := []seccomp.RuleSet{
 			// Trap mmap system calls and handle them in sigsysGoHandler