cc-wrapper: fix -Bprefix to not confuse lib/libc++.so and bin/c++

Before this commit, `pkgs/build-support/cc-wrapper/add-flags.sh`
was using `-B@out@/bin` instead of `-B@bintools@/bin`
to force `cc` to use `ld-wrapper.sh` when calling `ld`.
That was confusing `cc` when asked to print
the location of a library precisely named `c++`
because `-B` prefixes are also used by `cc` to find libraries,
see https://gcc.gnu.org/onlinedocs/gcc/Directory-Options.html#index-B

Indeed, instead of having `cc --print-file-name c++`
failing to found a `c++` library and just returning the given `c++` string
to let a linker resolve it thereafter,
it was finding that `@out@/bin/c++` executable,
mistaking it for a library and returning its absolute path,
forcing the linker to load an executable as a library.

Before this commit:

```console
$ nix run -f . stdenv.cc -- --print-file-name c++
/nix/store/9bv7dcvmfcjnmg5mnqwqlq2wxfn8d7yi-gcc-wrapper-13.2.0/bin/c++
```

After this commit:

```console
$ nix run -f . stdenv.cc -- --print-file-name c++
c++
```

Fixes https://gitlab.haskell.org/ghc/ghc/-/issues/23138#note_567034
where this behavior was breaking GHC on Darwin.

[Confirmed by @414owen](NixOS#317224 (comment)):

> This fixed all our haskell builds on Arm64 darwin, which were trying
> to link in clang++...

pkgs/build-support/cc-wrapper/add-flags.sh

@@ -29,8 +29,8 @@ done
 
 # Arocc does not support "-B"
 if [[ -z "@isArocc@" ]]; then
-    # `-B@out@/bin' forces cc to use ld-wrapper.sh when calling ld.
-    NIX_CFLAGS_COMPILE_@suffixSalt@="-B@out@/bin/ $NIX_CFLAGS_COMPILE_@suffixSalt@"
+    # `-B@bintools@/bin' forces cc to use ld-wrapper.sh when calling ld.
+    NIX_CFLAGS_COMPILE_@suffixSalt@="-B@bintools@/bin/ $NIX_CFLAGS_COMPILE_@suffixSalt@"
 fi
 
 # Export and assign separately in order that a failing $(..) will fail