Fix weak handling with GCC 3.4+

[Ulrich Drepper]

Andreas Jaeger wrote:
> As discussed yesterday, GCC 3.4 needs to be aware of weak symbols.
> I'm appending a patch that solves all problems I noticed.

The patch is partly OK and I've checked in the parts.  But the main part


> 	* include/libc-symbols.h (weak_extern): Use weak attribute instead
> 	of assembler so that GCC knows about it.

is broken.  The problem is this:

__thread int foo = 0;
extern __typeof (foo) foo __attribute ((weak));


__typeof() apparently does not include the __thread information.  I'd
call it a gcc bug.  This means:

a) we cannot use the new code unconditionally

b) gcc first has to be fixed before we can use a patch like this

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