This reverts commit 0a944b345e, reversing
changes made to 61733ed6cc.
I dislike these massive stdenv changes with unclear motivation,
especially when they involve gratuitous mass renames like NIX_CC ->
NIX_BINUTILS. The previous such rename (NIX_GCC -> NIX_CC) caused
months of pain, so let's not do that again.
cctool's as needs to be told use to use gnu as, or else we'd need a
dependency cycle between cctools and clang for this case.
In general, this is not a problem because clang uses its own integrated
assembler where possible, and gnu as otherwise.
