ld-wrapper: Put back the --sysroot filter

We still need this because some clang-based packages depend on
it. (The sysroot filtering was originally done by clang-wrapper's
ld-wrapper, but we merged the ld-wrappers in
a4f9b9c8b5ec9ef106671ffdf93e0059835d0ec1.)

http://hydra.nixos.org/build/13906922

pkgs/build-support/gcc-wrapper/ld-wrapper.sh

@@ -32,6 +32,9 @@ if test "$NIX_ENFORCE_PURITY" = "1" -a -n "$NIX_STORE" \
             # We cannot skip this; barf.
             echo "impure path \`$p' used in link" >&2
             exit 1
+        elif test "${p:0:9}" = "--sysroot"; then
+            # Our ld is not built with sysroot support (Can we fix that?)
+            :
         else
             rest=("${rest[@]}" "$p")
         fi