binutils-wrapper: Init

Factor a binutils wrapper out of cc-wrapper. While only LD is wrapped,
the setup hook defines environment variables on behalf of other
utilites.

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

@@ -13,6 +13,12 @@ fi
 
 source @out@/nix-support/utils.sh
 
+# Flirting with a layer violation here.
+if [ -z "${NIX_BINUTILS_WRAPPER_@infixSalt@_FLAGS_SET:-}" ]; then
+    source @binutils@/nix-support/add-flags.sh
+fi
+
+# Put this one second so libc ldflags take priority.
 if [ -z "${NIX_CC_WRAPPER_@infixSalt@_FLAGS_SET:-}" ]; then
     source @out@/nix-support/add-flags.sh
 fi