[ELF] Avoid creating a 2.1GB output file in arm-exidx-range.s

Currently both sections will be placed in the same PT_LOAD and therefore
lld generates a contiguous output file containing both sections.
By using AT(0xffff0000) the .vectors is placed a separate PT_LOAD and the
resulting file is now only a few kilobytes.

Reviewed By: psmith

Differential Revision: https://reviews.llvm.org/D85661