top level: Clean up definitions of alternative stdenvs

- Dispatch off more appropriate conditions---`stdenv.cc.is*` and
   `hostPlatform.is*` directly---rather than the OS as a proxy.

 - Don't worry about pulling in binutils from normal `stdenv.cc` for
   `gccMultiStdenv`.

 - Defining a `multiStdenv` that uses whatever compiler is default.

 - Define `stdenv_32bit` in terms of `multiStdenv`.

pkgs/top-level/all-packages.nix

@@ -34,11 +34,7 @@ with pkgs;
   # A stdenv capable of building 32-bit binaries.  On x86_64-linux,
   # it uses GCC compiled with multilib support; on i686-linux, it's
   # just the plain stdenv.
-  stdenv_32bit = lowPrio (
-    if system == "x86_64-linux" then
-      overrideCC stdenv gcc_multi
-    else
-      stdenv);
+  stdenv_32bit = lowPrio (if hostPlatform.is32bit then stdenv else multiStdenv);
 
   stdenvNoCC = stdenv.override { cc = null; };
 
@@ -5544,7 +5540,7 @@ with pkgs;
   };
 
   #Use this instead of stdenv to build with clang
-  clangStdenv = if stdenv.isDarwin then stdenv else lowPrio llvmPackages.stdenv;
+  clangStdenv = if stdenv.cc.isClang then stdenv else lowPrio llvmPackages.stdenv;
   clang-sierraHack-stdenv = overrideCC stdenv clang-sierraHack;
   libcxxStdenv = if stdenv.isDarwin then stdenv else lowPrio llvmPackages.libcxxStdenv;
 
@@ -5601,11 +5597,12 @@ with pkgs;
   gcc = gcc6;
   gcc-unwrapped = gcc.cc;
 
-  gccStdenv = if (!stdenv.isDarwin) then stdenv else stdenv.override {
+  gccStdenv = if stdenv.cc.isGNU then stdenv else stdenv.override {
     allowedRequisites = null;
     cc = gcc;
-    # Include unwrapped binaries like AS, etc. and remove libcxx/libcxxabi
-    extraBuildInputs = [ stdenv.cc.cc ];
+    # Remove libcxx/libcxxabi, and add clang for AS if on darwin (it uses
+    # clang's internal assembler).
+    extraBuildInputs = lib.optional hostPlatform.isDarwin clang.cc;
   };
 
   wrapCCMulti = cc:
@@ -5645,6 +5642,7 @@ with pkgs;
 
   gccMultiStdenv = overrideCC stdenv gcc_multi;
   clangMultiStdenv = overrideCC stdenv clang_multi;
+  multiStdenv = if stdenv.cc.isClang then clangMultiStdenv else gccMultiStdenv;
 
   gcc_debug = lowPrio (wrapCC (gcc.cc.override {
     stripped = false;