Also remove the hard-coded path for getting glibc information.

As an example, the following is the diff for `go bug` on Ubuntu before
and after the change:

>>>
--- /tmp/01     2016-09-13 15:08:53.202758043 +0530
+++ /tmp/02     2016-09-13 21:38:17.485039867 +0530
@@ -1,7 +1,7 @@
 Please check whether the issue also reproduces on the latest release, go1.7.1.

 ```
-go version devel +bdb3b790 Wed Sep 7 03:23:44 2016 +0000 linux/amd64
+go version devel +cb13150 Wed Sep 7 09:46:58 2016 +0530 linux/amd64
 GOARCH="amd64"
 GOBIN=""
 GOEXE=""
@@ -18,5 +18,23 @@
 CXX="g++"
 CGO_ENABLED="1"
 uname -sr: Linux 4.4.0-36-generic
+Distributor ID:        Ubuntu
+Description:   Ubuntu 16.04.1 LTS
+Release:       16.04
+Codename:      xenial
+/lib/x86_64-linux-gnu/libc.so.6: GNU C Library (Ubuntu GLIBC 2.23-0ubuntu3) stable release version 2.23, by Roland McGrath et al.
 gdb --version: GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.04) 7.11.1
 ```
<<<

Change-Id: I7e3730a797af0f94d6e43fe4743ab48bc0f11f1b
Reviewed-on: https://go-review.googlesource.com/28581
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>