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 <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/syscall_unix.go

diff --git a/unix/syscall_unix.go b/unix/syscall_unix.go
index 3de3756..8f710d0 100644
--- a/unix/syscall_unix.go
+++ b/unix/syscall_unix.go
@@ -76,7 +76,7 @@
 // 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
 		}