Use the plt entry as the address of some symbols.

This is the function equivalent of a copy relocation.

Since functions are expected to change sizes, we cannot use copy
relocations. In situations where one would be needed, what is done
instead is:
* Create a plt entry
* Output an undefined symbol whose addr is the plt entry.

The dynamic linker makes sure any shared library uses the plt entry as
the function address.

llvm-svn: 260224