[ARM, ELF] Don't shift movt relocation offsets

For ELF, a movw+movt pair is handled as two separate relocations.
If an offset should be applied to the symbol address, this offset is
stored as an immediate in the instruction (as opposed to stored as an
offset in the relocation itself).

Even though the actual value stored in the movt immediate after linking
is the top half of the value, we need to store the unshifted offset
prior to linking. When the relocation is made during linking, the offset
gets added to the target symbol value, and the upper half of the value
is stored in the instruction.

This makes sure that movw+movt with offset symbols get properly
handled, in case the offset addition in the lower half should be
carried over to the upper half.

This makes the output from the additions to the test case match
the output from GNU binutils.

For COFF and MachO, the movw/movt relocations are handled as a pair,
and the overflow from the lower half gets carried over to the movt,
so they should keep the shifted offset just as before.

Differential Revision: https://reviews.llvm.org/D35242

llvm-svn: 307713