[patch] Use $target_subdir in a more normal way in gcc

[Nathanael Nerode]

>> This changes target_subdir in gcc to be compatible with target_subdir as used
>> at the top level (where it is always present).  No behavior changes.
>
>If target_subdir is always target_alias, why do we need both?

Because we might want to change it in the future. :-)  I can actually 
think of good reasons to, but none of them are applicable at the moment.

My intention, after some renaming, is to get gcc using 
GCC_TOPLEV_SUBDIRS just like the top level.  It's the irritating 
renaming that I'm doing now.

--Nathanael