treewide: Try to make a few bootstrapping things more consistent

- Introduce `preLibcCrossHeaders` to bootstrap libgcc and compiler-rt
  the same way.

- Organize LLVM bintools as `bintools{-unwrapped,,NoLibc}` for
  consistency with GNU Binutils and Apple's cctools.

- Do Android changes for all `llvmPackages` for consistency.

- Improve the way the default GCC and LLVM versions are selected.

pkgs/development/compilers/llvm/9/bintools/default.nix

@@ -0,0 +1,29 @@
+{ runCommand, stdenv, llvm, lld, version }:
+
+let
+  prefix =
+    if stdenv.hostPlatform != stdenv.targetPlatform
+    then "${stdenv.targetPlatform.config}-"
+    else "";
+in runCommand "llvm-binutils-${version}" { preferLocalBuild = true; } ''
+   mkdir -p $out/bin
+   for prog in ${lld}/bin/*; do
+     ln -s $prog $out/bin/${prefix}$(basename $prog)
+   done
+   for prog in ${llvm}/bin/*; do
+     ln -sf $prog $out/bin/${prefix}$(basename $prog)
+   done
+
+   ln -s ${llvm}/bin/llvm-ar $out/bin/${prefix}ar
+   ln -s ${llvm}/bin/llvm-as $out/bin/${prefix}as
+   ln -s ${llvm}/bin/llvm-dwp $out/bin/${prefix}dwp
+   ln -s ${llvm}/bin/llvm-nm $out/bin/${prefix}nm
+   ln -s ${llvm}/bin/llvm-objcopy $out/bin/${prefix}objcopy
+   ln -s ${llvm}/bin/llvm-objdump $out/bin/${prefix}objdump
+   ln -s ${llvm}/bin/llvm-ranlib $out/bin/${prefix}ranlib
+   ln -s ${llvm}/bin/llvm-readelf $out/bin/${prefix}readelf
+   ln -s ${llvm}/bin/llvm-size $out/bin/${prefix}size
+   ln -s ${llvm}/bin/llvm-strip $out/bin/${prefix}strip
+
+   ln -s ${lld}/bin/lld $out/bin/${prefix}ld
+''