Undefined weak symbols are represented by a dummy sym_zero definition in
RTLD. Correctly handle this special case when creating a trampoline.

Non-PLT functions must have a canonical address so that different
libraries see the same address for the same function.

Previously, some but not all non-PLT functions are wrapped in
trampolines, breaking function pointer equality. For example, relative
relocations of function pointers are not wrapped in trampolines at all.

This commit completely disables trampolines for non-PLT relocations
until a complete solution for wrapping all function pointers is
implemented.

This makes functions such as dladdr work even when given a trampoline.