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/12/compiler-rt/default.nix

@@ -9,7 +9,7 @@ let
 
 in
 
-stdenv.mkDerivation rec {
+stdenv.mkDerivation {
   pname = "compiler-rt" + lib.optionalString (haveLibc) "-libc";
   inherit version;
   src = fetch "compiler-rt" "0d444qihq9jhqnfv003cr704v363va72zl6qaw2algj1c85cva45";
@@ -61,7 +61,6 @@ stdenv.mkDerivation rec {
   ]# ++ lib.optional stdenv.hostPlatform.isMusl ./sanitizers-nongnu.patch
     ++ lib.optional stdenv.hostPlatform.isAarch32 ./armv7l.patch;
 
-
   # TSAN requires XPC on Darwin, which we have no public/free source files for. We can depend on the Apple frameworks
   # to get it, but they're unfree. Since LLVM is rather central to the stdenv, we patch out TSAN support so that Hydra
   # can build this. If we didn't do it, basically the entire nixpkgs on Darwin would have an unfree dependency and we'd

pkgs/development/compilers/llvm/12/default.nix

@@ -1,4 +1,5 @@
-{ lowPrio, newScope, pkgs, lib, stdenv, cmake, gccForLibs
+{ lowPrio, newScope, pkgs, lib, stdenv, cmake
+, gccForLibs, preLibcCrossHeaders
 , libxml2, python3, isl, fetchurl, overrideCC, wrapCCWith, wrapBintoolsWith
 , buildLlvmTools # tools, but from the previous stage, for cross
 , targetLlvmLibraries # libraries, but from the next stage, for cross
@@ -112,7 +113,16 @@ let
     # doesn’t support like LLVM. Probably we should move to some other
     # file.
 
-    bintools = callPackage ./bintools {};
+    bintools-unwrapped = callPackage ./bintools {};
+
+    bintoolsNoLibc = wrapBintoolsWith {
+      bintools = tools.bintools-unwrapped;
+      libc = preLibcCrossHeaders;
+    };
+
+    bintools = wrapBintoolsWith {
+      bintools = tools.bintools-unwrapped;
+    };
 
     lldClang = wrapCCWith rec {
       cc = tools.clang-unwrapped;
@@ -141,9 +151,7 @@ let
     lldClangNoLibcxx = wrapCCWith rec {
       cc = tools.clang-unwrapped;
       libcxx = null;
-      bintools = wrapBintoolsWith {
-        inherit (tools) bintools;
-      };
+      inherit (tools) bintools;
       extraPackages = [
         targetLlvmLibraries.compiler-rt
       ];
@@ -157,10 +165,7 @@ let
     lldClangNoLibc = wrapCCWith rec {
       cc = tools.clang-unwrapped;
       libcxx = null;
-      bintools = wrapBintoolsWith {
-        inherit (tools) bintools;
-        libc = null;
-      };
+      bintools = tools.bintoolsNoLibc;
       extraPackages = [
         targetLlvmLibraries.compiler-rt
       ];
@@ -173,10 +178,7 @@ let
     lldClangNoCompilerRt = wrapCCWith rec {
       cc = tools.clang-unwrapped;
       libcxx = null;
-      bintools = wrapBintoolsWith {
-        inherit (tools) bintools;
-        libc = null;
-      };
+      bintools = tools.bintoolsNoLibc;
       extraPackages = [ ];
       extraBuildCommands = ''
         echo "-nostartfiles" >> $out/nix-support/cc-cflags
@@ -186,9 +188,7 @@ let
     lldClangNoCompilerRtWithLibc = wrapCCWith rec {
       cc = tools.clang-unwrapped;
       libcxx = null;
-      bintools = wrapBintoolsWith {
-        inherit (tools) bintools;
-      };
+      inherit (tools) bintools;
       extraPackages = [ ];
       extraBuildCommands = mkExtraBuildCommands0 cc;
     };
@@ -199,15 +199,19 @@ let
     callPackage = newScope (libraries // buildLlvmTools // { inherit stdenv cmake libxml2 python3 isl release_version version fetch; });
   in {
 
-    compiler-rt-libc = callPackage ./compiler-rt ({ inherit llvm_meta; } //
-      (lib.optionalAttrs (stdenv.hostPlatform.useLLVM or false) {
-        stdenv = overrideCC stdenv buildLlvmTools.lldClangNoCompilerRtWithLibc;
-      }));
+    compiler-rt-libc = callPackage ./compiler-rt {
+      inherit llvm_meta;
+      stdenv = if stdenv.hostPlatform.useLLVM or false
+               then overrideCC stdenv buildLlvmTools.lldClangNoCompilerRtWithLibc
+               else stdenv;
+    };
 
-    compiler-rt-no-libc = callPackage ./compiler-rt ({ inherit llvm_meta; } //
-      (lib.optionalAttrs (stdenv.hostPlatform.useLLVM or false) {
-        stdenv = overrideCC stdenv buildLlvmTools.lldClangNoCompilerRt;
-      }));
+    compiler-rt-no-libc = callPackage ./compiler-rt {
+      inherit llvm_meta;
+      stdenv = if stdenv.hostPlatform.useLLVM or false
+               then overrideCC stdenv buildLlvmTools.lldClangNoCompilerRt
+               else stdenv;
+    };
 
     # N.B. condition is safe because without useLLVM both are the same.
     compiler-rt = if stdenv.hostPlatform.isAndroid