llvmPackages_7.clang: backport patch introducing --unwindlib flag

This flag was introduced for clang 9, but we use it in the `lldClang`
wrapper for `llvmPackages` 7, 8 and 9. For this purpose the patch was
backported for `llvmPackages_8.clang`, but not for `llvmPackages_7.clang`
which has been done in this commit.

`lldClang` is mostly used when cross compiling and
`stdenv.hostPlatform.useLLVM` is true. Most likely this problem wasn't
noticed since `useLLVM` with `llvmPackages_7` was broken for other
reasons as well and all cross targets (like `wasi32`) which have
`useLLVM` at the moment use `llvmPackages_8`.

With this change tests.cross.llvm.hello.{musl64, …} works again.

pkgs/development/compilers/llvm/7/clang/default.nix

@@ -43,6 +43,9 @@ let
       ./purity.patch
       # make clang -xhip use $PATH to find executables
       ./HIP-use-PATH-7.patch
+      # Backport for the `--unwindlib=[libgcc|compiler-rt]` flag, which is
+      # needed for our bootstrapping to not interfere with C.
+      ./unwindlib.patch
     ];
 
     postPatch = ''