tools/impact: Tweak the uname call

It turns out that OS X doesn't support long options for uname and it
doesn't support -o/--operating-system at all. Let's tweak uname's
options into something POSIX-compatible which still gives reasonable
results.

The new "uname -mrs" call results in the following:

  OS               uname -mrs
  -----------------------------------------------
  OS X Mavericks   Darwin 15.2.0 x86_64
  Ubuntu 14.04     Linux 3.13.0-32-generic x86_64

tools/impact

@@ -61,7 +61,7 @@ print_results() {
 
   echo
 
-  echo "(Measured by /tools/impact with Node.js $(node --version) on $(uname --operating-system --machine).)"
+  echo "(Measured by /tools/impact with Node.js $(node --version) on $(uname -mrs).)"
 }
 
 print_usage() {