unix: allocate signalNameMap with enough space

Avoid re-allocations while filling signalNameMap.

Change-Id: I3017e856c2724debccad43324cbe2e60b68d869c
Reviewed-on: https://go-review.googlesource.com/c/sys/+/225017
Run-TryBot: Tobias Klauser <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

unix/syscall_unix.go

@@ -76,7 +76,7 @@ func SignalName(s syscall.Signal) string {
 // The signal name should start with "SIG".
 func SignalNum(s string) syscall.Signal {
 	signalNameMapOnce.Do(func() {
-		signalNameMap = make(map[string]syscall.Signal)
+		signalNameMap = make(map[string]syscall.Signal, len(signalList))
 		for _, signal := range signalList {
 			signalNameMap[signal.name] = signal.num
 		}