bug#32131: ghc fails to compile packages when installed into profile

[Anonymous]

This bug can be reproduced by doing

    guix package -i ghc
    echo 'main=return()' > test.hs
    ghc test.hs

The expected output is that the program will be compiled, but instead we 
get the error message

    gcc: error trying to exec 'as': execvp: No such file or directory
    `gcc' failed in phase `C Compiler'. (Exit code: 1)

The error is caused, of course, by as not being found in the path. 
Interestingly, though, the error occurs in gcc, which fails to find as. 
The general path for compiling things with gcc is to use gcc-toolchain, 
which brings as and others into the profile, but as ghc calls gcc 
directly (with no gcc-toolchain), ghc fails to actually compile anything.