rt: Define exp10 on Android

LLVM appears to generate calls to exp10 on ARM and bionic does not
define it.
auto merge of #11299 : brson/rust/exp10, r=pcwalton

LLVM appears to generate calls to exp10 on ARM and bionic does not define it.

This makes code that links to libextra (which I guess does some exponentiation on the stat module) link correctly.