proc: fix stacktraces on freebsd/amd64/go1.20

TestStacktraceGoroutine failed intermittently on freebsd/amd64/go1.20
due to stacktraces being broken when doing a runtime.nanotime system
call.

pkg/proc/amd64_arch.go

@@ -200,11 +200,19 @@ func amd64SwitchStack(it *stackIterator, _ *op.DwarfRegisters) bool {
 
 		return true
 
-	case "runtime.goexit", "runtime.rt0_go", "runtime.mcall":
+	case "runtime.goexit", "runtime.rt0_go":
 		// Look for "top of stack" functions.
 		it.atend = true
 		return true
 
+	case "runtime.mcall":
+		if it.systemstack && it.g != nil && it.bi.GOOS == "freebsd" {
+			it.switchToGoroutineStack()
+			return true
+		}
+		it.atend = true
+		return true
+
 	case "runtime.mstart":
 		// Calls to runtime.systemstack will switch to the systemstack then:
 		// 1. alter the goroutine stack so that it looks like systemstack_switch