Until a few weeks ago, bionic, the Andoid libc, incorrectly
returned const char* (instead of char*) from dlerror(3).

https://android.googlesource.com/platform/bionic/+/5e071a18ce88d93fcffaebb9e0f62524ae504908

Change-Id: I30d33240c63a9f35b6c20ca7e3928ad33bc5e33f
Reviewed-on: https://go-review.googlesource.com/29352
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>