Add toolchain wrapper for external toolchains

Add a simple toolchain wrapper for external toolchains, which forces the
correct sysroot/march/mtune/floating point options needed to use it
with buildroot.

With this in place the external toolchain behaves similar to the internal
ones, and the special handling can be removed. This also means that the
toolchain is usable outside buildroot without having to pass any special
compiler flags.

Also adjust the downloadable external toolchain support to install under
HOST_DIR so it can be used after the temporary build files are removed.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>