log: add global Output function

It was just an oversight that this one method of Logger was not
made available for the standard (std) Logger.

Fixes #9183

Change-Id: I2f251becdb0bae459212d09ea0e5e88774d16dea
Reviewed-on: https://go-review.googlesource.com/2686Reviewed-by: Andrew Gerrand <adg@golang.org>

src/log/log.go

@@ -322,3 +322,14 @@ func Panicln(v ...interface{}) {
 	std.Output(2, s)
 	panic(s)
 }
+
+// Output writes the output for a logging event.  The string s contains
+// the text to print after the prefix specified by the flags of the
+// Logger.  A newline is appended if the last character of s is not
+// already a newline.  Calldepth is the count of the number of
+// frames to skip when computing the file name and line number
+// if Llongfile or Lshortfile is set; a value of 1 will print the details
+// for the caller of Output.
+func Output(calldepth int, s string) error {
+	return std.Output(calldepth+1, s) // +1 for this frame.
+}