The currenr ELF visibility handling is wrong on several things:

1. Ld doesn't report an error for undefined symbols with non-default
visibility when there is a definition from a shared library.
2. Weak undefined symbols with non-default visibility results in
undetermined run-time behavior depending on if there is a definition
at run-time.
3. Ld gives no warnings when a shared library refeneces a hidden symbol.
4. ld merges visibility with symbols from shared libraries.

This patch fixes those problems with testcases. As for weak undefined 
symbols with non-default visibility, I only fixed x86 and ia64.



H.J.