buildman "ambiguous toolchains" for host-arch == target arch

When downloading a cross-toolchain for the host arch, we get:

Warning, ambiguous toolchains: .../${host}-linux-${host}-linux-gcc, .../${host}-linux-gcc

And this has always been true. It's not a problem for amd64 in that we don't use the "x86_64" toolchain for CONFIG_X86 platforms but on arm64 this means that we get for example:

aarch64   : /opt/gcc-13.2.0-nolibc/aarch64-linux/bin/aarch64-linux-aarch64-linux-gcc

and then fail to link because aarch64-linux-aarch64-ld doesn't (and shouldn't) exist. We can work-around this for now with a toolchain-prefix section.