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/compiler-rt/default.nix

@@ -4,14 +4,15 @@ let
 
   useLLVM = stdenv.hostPlatform.useLLVM or false;
   bareMetal = stdenv.hostPlatform.parsed.kernel.name == "none";
+  haveLibc = stdenv.cc.libc != null;
   inherit (stdenv.hostPlatform) isMusl;
 
 in
 
-stdenv.mkDerivation rec {
-  pname = "compiler-rt";
+stdenv.mkDerivation {
+  pname = "compiler-rt" + lib.optionalString (haveLibc) "-libc";
   inherit version;
-  src = fetch pname "0xwh79g3zggdabxgnd0bphry75asm1qz7mv3hcqihqwqr6aspgy2";
+  src = fetch "compiler-rt" "0xwh79g3zggdabxgnd0bphry75asm1qz7mv3hcqihqwqr6aspgy2";
 
   nativeBuildInputs = [ cmake python3 llvm.dev ];
   buildInputs = lib.optional stdenv.hostPlatform.isDarwin libcxxabi;
@@ -29,14 +30,15 @@ stdenv.mkDerivation rec {
     "-DCOMPILER_RT_BUILD_XRAY=OFF"
     "-DCOMPILER_RT_BUILD_LIBFUZZER=OFF"
     "-DCOMPILER_RT_BUILD_PROFILE=OFF"
-  ] ++ lib.optionals (useLLVM || bareMetal) [
+  ] ++ lib.optionals ((useLLVM || bareMetal) && !haveLibc) [
     "-DCMAKE_C_COMPILER_WORKS=ON"
     "-DCMAKE_CXX_COMPILER_WORKS=ON"
     "-DCOMPILER_RT_BAREMETAL_BUILD=ON"
     "-DCMAKE_SIZEOF_VOID_P=${toString (stdenv.hostPlatform.parsed.cpu.bits / 8)}"
+  ] ++ lib.optionals (useLLVM && !haveLibc) [
+    "-DCMAKE_C_FLAGS=-nodefaultlibs"
   ] ++ lib.optionals (useLLVM) [
     "-DCOMPILER_RT_BUILD_BUILTINS=ON"
-    "-DCMAKE_C_FLAGS=-nodefaultlibs"
     #https://stackoverflow.com/questions/53633705/cmake-the-c-compiler-is-not-able-to-compile-a-simple-test-program
     "-DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY"
   ] ++ lib.optionals (bareMetal) [