useLdGold previously just checked for useLLVM which (currently) implies
`linker == "lld"`. However more accurate is to check the `linker` of the
`targetPlatform` as it actually tells us which bintools package we can
expect.
`linker == "bfd"` implies that we are using the `binutils` package, so
gold is available, so we can use it unless musl is the libc. `linker ==
"gold"` implies that gold is the default linker already and we should
absolutely use it.
GHC calls otool on darwin which is contained in the
stdenv.cc.bintools.bintools derivation and thus needs adding to the
runtime PATH of GHC. Since this is toolchain specific technically, we
check for cctools instead of darwin (although I don't know if GHC
or nixpkgs work on macOS without cctools).
This fixes usage of GHC in an environment where otool is not available
and more specifically in stdenvNoCC which is used by writers.writeHaskell.
Resolves#123228.
