rustc: fix build on darwin

The OS is identified as "10.4" rather than "osx". This commit removes
the 'rt' build script's hard coding of the "osx" suffix when building on
darwin since the cmake configuration falls back to building a 10.4
compatible libclang_rt.

pkgs/development/compilers/rust/rustc.nix

@@ -92,6 +92,13 @@ stdenv.mkDerivation {
 
     # Useful debugging parameter
     # export VERBOSE=1
+  '' +
+  # In src/compiler-rt/cmake/config-ix.cmake, the cmake build falls
+  # back to darwin 10.4. This causes the OS name to be recorded as
+  # "10.4" rather than the expected "osx". But mk/rt.mk expects the
+  # built library name to have an "_osx" suffix on darwin.
+  optionalString stdenv.isDarwin ''
+    substituteInPlace mk/rt.mk --replace "_osx" "_10.4"
   '';
 
   preConfigure = ''
@@ -118,7 +125,13 @@ stdenv.mkDerivation {
 
   preCheck = ''
     export TZDIR=${tzdata}/share/zoneinfo
-    ${optionalString stdenv.isDarwin "export TMPDIR=/tmp"}
+  '' +
+  # Ensure TMPDIR is set, and disable a test that removing the HOME
+  # variable from the environment falls back to another home
+  # directory.
+  optionalString stdenv.isDarwin ''
+    export TMPDIR=/tmp
+    sed -i '28s/home_dir().is_some()/true/' ./src/test/run-pass/env-home-dir.rs
   '';
 
   # Disable doCheck on Darwin to work around upstream issue