cc-wrapper: add libcxxabi include flag for LLVM

Removed workaround from llvm 16.

Fixes including cxxabi.h on llvm >=15 libcxxStdenv.

```c
int main() {}
```

```
/nix/store/qwnvng0cbyx0bijm654jpmpl0516hfhx-libcxxabi-15.0.7-dev/include/cxxabi.h:20:10: fatal error: '__cxxabi_config.h' file not found
```

Before llvm 15 this used to work because `libcxx` copied the headers
from `cxxabi` to it's own `include`, which was then picked up by the
line above this one

Alternative fix would be to copy all files from `${cxxabi.dev}/include/c++/v1` to `${cxxabi.dev}/include` so the cc-wrapper setup hook would pick them up, but that would depend on in cxxabi being in buildInputs.

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

pkgs/build-support/cc-wrapper/default.nix

@@ -469,6 +469,7 @@ stdenv.mkDerivation {
     ''
     + optionalString (libcxx.isLLVM or false) ''
       echo "-isystem ${lib.getDev libcxx}/include/c++/v1" >> $out/nix-support/libcxx-cxxflags
+      echo "-isystem ${lib.getDev libcxx.cxxabi}/include/c++/v1" >> $out/nix-support/libcxx-cxxflags
       echo "-stdlib=libc++" >> $out/nix-support/libcxx-ldflags
       echo "-l${libcxx.cxxabi.libName}" >> $out/nix-support/libcxx-ldflags
     ''

pkgs/development/compilers/llvm/16/default.nix

@@ -255,14 +255,6 @@ in let
         [ "-rtlib=compiler-rt"
           "-Wno-unused-command-line-argument"
           "-B${targetLlvmLibraries.compiler-rt}/lib"
-
-          # Combat "__cxxabi_config.h not found". Maybe this could be fixed by
-          # copying these headers into libcxx? Note that building libcxx
-          # outside of monorepo isn't supported anymore, might be related to
-          # https://github.com/llvm/llvm-project/issues/55632
-          # ("16.0.3 libcxx, libcxxabi: circular build dependencies")
-          # Looks like the machinery changed in https://reviews.llvm.org/D120727.
-          "-I${lib.getDev targetLlvmLibraries.libcxx.cxxabi}/include/c++/v1"
         ]
         ++ lib.optional (!stdenv.targetPlatform.isWasm) "--unwindlib=libunwind"
         ++ lib.optional