When constructing global address literals while targeting the RWPI relocation model. LLVM currently only uses literal pools. If MOVW/MOVT instructions are available we can use these instead. Beside being more efficient it allows -arm-execute-only to work with -relocation-model=RWPI as well.
When we generate MOVW/MOVT for global addresses when targeting the RWPI relocation model, we need to use base relative relocations. This patch does the needed plumbing in MC to generate these for MOVW/MOVT.