Handle undefined weak hidden symbols on all architectures.

We were handling the non-hidden case in lib/Target/TargetMachine.cpp,
but the hidden case was handled in architecture dependent code and
only X86_64 and AArch64 were covered.

While it is true that some code sequences in some ABIs might be able
to produce the correct value at runtime, that doesn't seem to be the
common case.

I left the AArch64 code in place since it also forces a got access for
non-pic code. It is not clear if that is needed, but it is probably
better to change that in another commit.

llvm-svn: 316799