Android toolchain: add ANDROID_COMPILER_PREFIX variable.

Summary:
For ARM it's not necessary, but when building for a x86 tablet I had to
set ANDROID_TOOLCHAIN=x86 while gcc/g++ are prefixed with
i686-linux-android.

i.e. the path is android-ndk-r10e/toolchains/x86-4.9/prebuilt/linux-x86_64/bin/i686-linux-android-g++

Test Plan: cmake picks up the right compiler for me now

Reviewers: apol, mart

Reviewed By: apol

Subscribers: Build System, Frameworks

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D10462