linker: rom_start_offset: add to address instead of set

The CONFIG_ROM_START_OFFSET is supposed to be added to
the current when linking, instead of having the current
address set to it. So fix that.

Not sure why it worked up to this point, but llvm/clang/lld
complained that it could not move location counter backward.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>