[ELF] Do not create interworking thunks for undefined weak references.

An undefined weak reference is given an address of 0 this will
incorrectly trigger the creation of a Thumb to ARM interworking Thunk
if there is a Thumb branch instruction to the symbol. This results in
an error as Thunks only make sense to defined or shared symbols.

We prevent this by detecting an undefined symbol and not creating a thunk
for it.

Differential Revision: https://reviews.llvm.org/D26239

llvm-svn: 285896

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