Extend the Binutils ports to allow deletion of specific tools.

For CloudABI I slowly want to switch away from certain tools provided by
Binutils. For example, it would be really nice if we could switch from
GNU ld to LLD some time in the future. LLD is not ready yet. Some bug
fixes didn't manage to land into 3.8 and it doesn't support aarch64
properly. There are already some tools that we can use, such as nm,
objdump, size, ar and ranlib.

Introduce a BUREMOVE variable that can be set in Binutils slave ports to
remove specific utilities that are installed by default. There doesn't
seem to be any other elegant way to solve this. Set a bunch of utilities
by default and then extend cloudabi-clang to set up symlinks to the LLVM
versions of the tools.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D5684