IMPALA-2960: ASAN compile error: cmath's log2 is not constexpr.
IMPALA-2960: ASAN compile error: cmath's log2 is not constexpr.

For details, see the C++ standard committee's library working group
issue #2013:

This was allowed in c++11:

http://lwg.github.io/issues/lwg-active.html#2013

But not in c++14:

http://cplusplus.github.io/LWG/lwg-defects.html#2013

Builtins like __builtin_clz are allowed to be constexpr.

Change-Id: Ibeaec9526fc9cc4d6c36bd82a1645369f1d7698e
Reviewed-on: http://gerrit.cloudera.org:8080/2090
Reviewed-by: Dan Hecht <20ebd99959f0f6e5dcb0fc319b4d8ef53a16b5fb@cloudera.com>
Tested-by: Internal Jenkins

ibmsoe/ImpalaPPC,ibmsoe/ImpalaPPC,ibmsoe/ImpalaPPC,ibmsoe/ImpalaPPC,ibmsoe/ImpalaPPC,ibmsoe/ImpalaPPC,ibmsoe/ImpalaPPC