Store the real binding of shared symbols.

Currently we mark every shared symbol as STB_WEAK.

That is a hack to make it easy to decide when a .so is needed or not
because of a reference to a given symbol.

That hack leaks when we create copy relocations as shown by the update
to relocation-copy-alias.s.

This patch stores the original binding when we first read a shared
symbol. We still have to update the binding to weak if we see a weak
undef, but I find the logic easier to read where it is now.

llvm-svn: 319127