bintools-wrapper: Init

Factor a bintools (i.e. binutils / cctools) 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

@@ -17,6 +17,12 @@ fi
 
 source @out@/nix-support/utils.sh
 
+# Flirting with a layer violation here.
+if [ -z "${NIX_BINTOOLS_WRAPPER_@infixSalt@_FLAGS_SET:-}" ]; then
+    source @bintools@/nix-support/add-flags.sh
+fi
+
+# Put this one second so libc ldflags take priority.
 if [ -z "${NIX_CC_WRAPPER_@infixSalt@_FLAGS_SET:-}" ]; then
     source @out@/nix-support/add-flags.sh
 fi