mingw-w64-cross-compiler-rt: fix compiler target
mingw-w64-cross-compiler-rt: fix compiler target

Instead of using `${CARCH}`, extract from `${_target}` in case they
should be different.

atom2013/MSYS2-packages,waterlan/MSYS2-packages,waterlan/MSYS2-packages,atom2013/MSYS2-packages,atom2013/MSYS2-packages,Alexpux/MSYS2-packages,Alexpux/MSYS2-packages,JPeterMugaas/MSYS2-packages,waterlan/MSYS2-packages,mati865/MSYS2-packages,mati865/MSYS2-packages,JPeterMugaas/MSYS2-packages,JPeterMugaas/MSYS2-packages,waterlan/MSYS2-packages,Alexpux/MSYS2-packages,atom2013/MSYS2-packages,Alexpux/MSYS2-packages,mati865/MSYS2-packages,mati865/MSYS2-packages,JPeterMugaas/MSYS2-packages