IA-64 gp problem with PROVIDE and linker relaxation conflict

[Jim Wilson]

Changing the tree node_class didn't occur to me.  If the trees aren't shared
then this could work, though it seems a little inelegant.  I don't see anyplace
else that changes the tree node_class after a tree is created.

I was thinking of maybe putting something in the h->u.def structure to
indicate that this was a "provided" value, and thus should be overriden by
another provide, but I didn't see anyplace convenient.

I did confirm that the patch works for my testcase.

Jim