Remove obsolete file
Add a stdenv adapter ‘useGoldLinker’ to force use of Gold
Apply makeOverridable to stdenv

This removes the need for hacks like stdenv.regenerate.  It also
ensures that overrideGCC is now stackable (so ‘stdenv = useGoldLinker
clangStdenv’ works).
stdenv: Simplify
addCoverageInstrumentation: Set NIX_CFLAGS_COMPILE as an attribute

This allows it to show up in nix-shell.
Turn the coverage analysis stdenv adapters into setup hooks

Stdenv adapters are kinda weird and un-idiomatic (especially when they
don't actually change stdenv).  It's more idiomatic to say

  buildInputs = [ makeCoverageAnalysisReport ];
lcov: Filter out /nix/store/* by default