rustc: fix build with unbundled jemalloc and llvm on darwin

jemalloc with stripped prefix would cause segfaults in free:
https://github.com/NixOS/nixpkgs/pull/49557#issuecomment-436734677

Thanks to @danieldk for darwin testing/debugging.

pkgs/development/libraries/jemalloc/common.nix

@@ -1,6 +1,13 @@
-{ stdenv, fetchurl, version, sha256, ... }@args:
+{ version, sha256 }:
+{ stdenv, fetchurl,
+# By default, jemalloc puts a je_ prefix onto all its symbols on OSX, which
+# then stops downstream builds (mariadb in particular) from detecting it. This
+# option should remove the prefix and give us a working jemalloc.
+# Causes segfaults with some software (ex. rustc), but defaults to true for backward
+# compatibility. Ignored on non OSX.
+stripPrefix ? true }:
 
-stdenv.mkDerivation (rec {
+stdenv.mkDerivation rec {
   name = "jemalloc-${version}";
   inherit version;
 
@@ -9,10 +16,8 @@ stdenv.mkDerivation (rec {
     inherit sha256;
   };
 
-  # By default, jemalloc puts a je_ prefix onto all its symbols on OSX, which
-  # then stops downstream builds (mariadb in particular) from detecting it. This
-  # option should remove the prefix and give us a working jemalloc.
-  configureFlags = stdenv.lib.optional stdenv.isDarwin "--with-jemalloc-prefix=";
+  # see the comment on stripPrefix
+  configureFlags = stdenv.lib.optional (stdenv.isDarwin && stripPrefix) "--with-jemalloc-prefix=";
   doCheck = true;
 
   enableParallelBuilding = true;
@@ -28,4 +33,4 @@ stdenv.mkDerivation (rec {
     platforms = platforms.all;
     maintainers = with maintainers; [ wkennington ];
   };
-} // (builtins.removeAttrs args [ "stdenv" "fetchurl" "version" "sha256" ]))
+}