bug#32773: [PATCH] Fix clang libstdc++ header location search

[David Truby <David.Truby@arm.com>]

Hi Mathieu,

> mathieu@elbruz ~/guix [env]$ ./pre-inst-env guix environment --ad-hoc 
> clang --pure
> mathieu@elbruz ~/guix [env]$ clang ~/tmp/t.cpp 
> clang-8: error: unable to execute command: Executable "ld" doesn't
> exist!
> clang-8: error: linker command failed with exit code 1 (use -v to see
> invocation)

Interesting, for some reason this worked for me. Although now that I
think about it I may not have used a pure environment, because I do
remember seeing this before. 

> 
> This is because the linker provided by "binutils" is not accessible.
> I
> guess we need to make something analog to make-gcc-toolchain.

I think adding gcc-toolchain to propagated-inputs should work in
principle, other than the fact that the gcc used for gcc-lib and
libstdc++ need to be the same as the propagated gcc-toolchain. I did
have a fix for this but I'm not sure if it's the best way of writing
it.
I'll send another patch with the fix though so at least others can
comment on it!

David Truby