[ARM] GlobalISel: Select globals in PIC mode

Support the selection of G_GLOBAL_VALUE in the PIC relocation model. For
simplicity we use the same pseudoinstructions for both Darwin and ELF:
(MOV|LDRLIT)_ga_pcrel(_ldr).

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.

Differential Revision: https://reviews.llvm.org/D36507

llvm-svn: 311992