runtime: print signal name in panic, if name is known

Adds a small function signame that infers a signal name
from the signal table, otherwise will fallback to using
hex(sig) as previously. No signal table is present for
Windows hence it will always print the hex value.

Sample code and new result:
```go
package main

import (
  "fmt"
  "time"
)

func main() {
  defer func() {
    if err := recover(); err != nil {
      fmt.Printf("err=%v\n", err)
    }
  }()

  ticker := time.Tick(1e9)
  for {
    <-ticker
  }
}
```

```shell
$ go run main.go &
$ kill -11 <pid>
fatal error: unexpected signal during runtime execution
[signal SIGSEGV: segmentation violation code=0x1 addr=0xb01dfacedebac1e
pc=0xc71db]
...
```

Fixes #13969

Change-Id: Ie6be312eb766661f1cea9afec352b73270f27f9d
Reviewed-on: https://go-review.googlesource.com/22753
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/signal_unix.go

@@ -12,3 +12,10 @@ import _ "unsafe" // for go:linkname
 func os_sigpipe() {
 	systemstack(sigpipe)
 }
+
+func signame(sig uint32) string {
+	if sig >= uint32(len(sigtable)) {
+		return ""
+	}
+	return sigtable[sig].name
+}