COFF ARM: Apply an existing offset in MOV32T relocations

Don't blindly OR in the new value, but clear the existing one, since it can be
nonzero. Read out the existing value before, and add into the desired offset.
(The add is done outside of the applyMOV, to handle potential overflow between
the two.)

Patch by Martin Storsjö!

llvm-svn: 277846