Support the selection of G_GLOBAL_VALUE in the PIC relocation model. For
simplicity we use the same pseudoinstructions for both Darwin and ELF:
LDRLIT_ga_pcrel(_ldr). On Darwin we also use MOV_ga_pcrel(_ldr) if
available, but not on ELF (see PR28229).
This is new for ELF, so it requires a small update to the ARM pseudo
expansion pass to make sure it adds the correct constant pool modifier
and add-current-address in the case of ELF.