9.3.0 Non-Native builds fail due to missing HOST_TRIPLE

[Jason-H]

On tag "gcc-9.3.0" commit 78bf864

In build-tools.sh, HOST_TRIPLE is only set for native builds, and is otherwise empty. This causes parameters to be shifted down one position, causing binutils build to fail:

binutils ${HOST_TRIPLE} ${BUILD_TRIPLE} ${TARGET_TRIPLE} "--enable-multilib"

[Lucretia]

I'm only partway through the process of making the toolchain build more generic.