Expose default std.log implementation

This allows root.log implementations to capture log messages, process them, and forward them to the default implementation if desired.

lib/std/log.zig

@@ -145,11 +145,26 @@ fn log(
             if (@typeInfo(@TypeOf(root.log)) != .Fn)
                 @compileError("Expected root.log to be a function");
             root.log(message_level, scope, format, args);
-        } else if (std.Target.current.os.tag == .freestanding) {
+        } else {
+            defaultLog(message_level, scope, format, args);
+        }
+    }
+}
+
+/// The default implementation for root.log.  root.log may forward log messages
+/// to this function.
+pub fn defaultLog(
+    comptime message_level: Level,
+    comptime scope: @Type(.EnumLiteral),
+    comptime format: []const u8,
+    args: anytype,
+) void {
+    if (std.Target.current.os.tag == .freestanding) {
         // On freestanding one must provide a log function; we do not have
         // any I/O configured.
         return;
-        } else {
+    }
+
     const level_txt = switch (message_level) {
         .emerg => "emergency",
         .alert => "alert",
@@ -165,8 +180,6 @@ fn log(
     const held = std.debug.getStderrMutex().acquire();
     defer held.release();
     nosuspend stderr.print(level_txt ++ prefix2 ++ format ++ "\n", args) catch return;
-        }
-    }
 }
 
 /// Returns a scoped logging namespace that logs all messages using the scope