llvmPackage_{3.4,3.5,3.7,3.8,3.9}: fix output of llvm-config

llvm-config is a tool to output compile and linker flags, when compiling against llvm.

The tool however outputs static library names despite libllvm is build
as shared library on nixos. This was fixed for llvm 3.4, 3.5 and 3.7.

For llvm 3.8 and 3.9 it printed the library extension twice (.so.so).
This was fixed in 4.0 and the patch is backported to 3.8 and 3.9 in
this pull request.

```
$ for i in 34 35 37 38 39; do echo "\nllvm-$i"; nix-shell -p llvmPackages_$i.llvm --run 'llvm-config --libnames'; done

llvm-34
libLLVMInstrumentation.so libLLVMIRReader.so libLLVMAsmParser.so
...

llvm-35
libLLVMLTO.so libLLVMObjCARCOpts.so libLLVMLinker.so libLLVMipo.so
...

llvm-37
libLLVMLTO.so libLLVMObjCARCOpts.so libLLVMLinker.so libLLVMBitWriter.so
...

llvm-38
libLLVM-3.8.1.so

llvm-39
libLLVM-3.9.so
```

fixes #26713

pkgs/development/compilers/llvm/3.8/fix-llvm-config.patch

@@ -0,0 +1,11 @@
+--- llvm.org/utils/llvm-build/llvmbuild/main.py	2015-10-07 00:24:35.000000000 +0100
++++ llvm/utils/llvm-build/llvmbuild/main.py	2017-06-16 17:08:39.866586578 +0100
+@@ -413,7 +413,7 @@
+             if library_name is None:
+                 library_name_as_cstr = 'nullptr'
+             else:
+-                library_name_as_cstr = '"lib%s.a"' % library_name
++                library_name_as_cstr = '"lib%s.so"' % library_name
+             if is_installed:
+                 is_installed_as_cstr = 'true'
+             else:

pkgs/development/compilers/llvm/3.8/llvm.nix

@@ -37,7 +37,8 @@ in stdenv.mkDerivation rec {
 
   # Fix a segfault in llc
   # See http://lists.llvm.org/pipermail/llvm-dev/2016-October/106500.html
-  patches = [ ./D17533-1.patch ];
+  patches = [ ./D17533-1.patch ] ++
+    stdenv.lib.optionals (!stdenv.isDarwin) [./fix-llvm-config.patch];
 
   # hacky fix: New LLVM releases require a newer OS X SDK than
   # 10.9. This is a temporary measure until nixpkgs darwin support is