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/7/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
@@ -95,7 +96,16 @@ let
     # doesn’t support like LLVM. Probably we should move to some other
     # file.
 
-    bintools = callPackage ./bintools.nix {};
+    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;
@@ -114,6 +124,8 @@ let
         echo "-B${targetLlvmLibraries.compiler-rt}/lib" >> $out/nix-support/cc-cflags
       '' + lib.optionalString (!stdenv.targetPlatform.isWasm) ''
         echo "--unwindlib=libunwind" >> $out/nix-support/cc-cflags
+      '' + lib.optionalString (stdenv.targetPlatform.isAndroid && stdenv.targetPlatform.useLLVM) ''
+        echo "-lunwind" >> $out/nix-support/cc-ldflags
       '' + lib.optionalString stdenv.targetPlatform.isWasm ''
         echo "-fno-exceptions" >> $out/nix-support/cc-cflags
       '' + mkExtraBuildCommands cc;
@@ -122,9 +134,7 @@ let
     lldClangNoLibcxx = wrapCCWith rec {
       cc = tools.clang-unwrapped;
       libcxx = null;
-      bintools = wrapBintoolsWith {
-        inherit (tools) bintools;
-      };
+      inherit (tools) bintools;
       extraPackages = [
         targetLlvmLibraries.compiler-rt
       ];
@@ -138,10 +148,7 @@ let
     lldClangNoLibc = wrapCCWith rec {
       cc = tools.clang-unwrapped;
       libcxx = null;
-      bintools = wrapBintoolsWith {
-        inherit (tools) bintools;
-        libc = null;
-      };
+      bintools = tools.bintoolsNoLibc;
       extraPackages = [
         targetLlvmLibraries.compiler-rt
       ];
@@ -154,28 +161,44 @@ 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
       '' + mkExtraBuildCommands0 cc;
     };
 
+    lldClangNoCompilerRtWithLibc = wrapCCWith rec {
+      cc = tools.clang-unwrapped;
+      libcxx = null;
+      inherit (tools) bintools;
+      extraPackages = [ ];
+      extraBuildCommands = mkExtraBuildCommands0 cc;
+    };
+
   });
 
   libraries = lib.makeExtensible (libraries: let
     callPackage = newScope (libraries // buildLlvmTools // { inherit stdenv cmake libxml2 python3 isl release_version version fetch; });
   in {
 
-    compiler-rt = callPackage ./compiler-rt {
+    compiler-rt-libc = callPackage ./compiler-rt {
+      stdenv = if stdenv.hostPlatform.useLLVM or false
+               then overrideCC stdenv buildLlvmTools.lldClangNoCompilerRtWithLibc
+               else stdenv;
+    };
+
+    compiler-rt-no-libc = callPackage ./compiler-rt {
       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
+      then libraries.compiler-rt-libc
+      else libraries.compiler-rt-no-libc;
+
     stdenv = overrideCC stdenv buildLlvmTools.clang;
 
     libcxxStdenv = overrideCC stdenv buildLlvmTools.libcxxClang;