Read "Target compiler" values from platform configs

As we switch the compiler which we use from Android's SDK in Qt 5.12 (from GCC to clang), we need a way to request that compiler from platform configs. The 'compiler' configuration given in the platform_configs was the 'host compiler' used to create the host tools. We also used that value in Android builds to actually define the 'target compiler' which was GCC in the past.

Because in Windows we need to define the host compiler (mingw or msvc) and the Android compiler (GCC or clang) and this also depends on the Qt5 branch, it's most convenient to just add the needed column (target compiler) and get rid of any assumptions in the code.