stdenv: Remove the special handling of gcc

Now gcc is just another build input, making it possible in the future
to have a stdenv that doesn't depend on a C compiler. This is very
useful on NixOS, since it would allow trivial builders like
writeTextFile to work without pulling in the C compiler.

pkgs/stdenv/generic/builder.sh

@@ -10,7 +10,6 @@ echo "$preHook" > $out/setup
 cat "$setup" >> $out/setup
 
 sed -e "s^@initialPath@^$initialPath^g" \
-    -e "s^@gcc@^$gcc^g" \
     -e "s^@shell@^$shell^g" \
     < $out/setup > $out/setup.tmp
 mv $out/setup.tmp $out/setup