Remove log2 dependency when building against Android API-9 SDK

Android's API-9 SDK is missing log2 builtins. A previous commit added
support for building against this API revision but this requires log2l
to be present.  (And it doesn't seem to be defined, despite being in
the headers.)

Author: pasaulais (Pierre-Andre Saulais)

Differential Revision: http://reviews.llvm.org/D9884

llvm-svn: 238111