[ARM] GlobalISel: Support global variables for RWPI

[ARM] GlobalISel: Support global variables for RWPI

In RWPI code, globals that are not read-only are accessed relative to
the SB register (R9). This is achieved by explicitly generating an ADD
instruction between SB and an offset that we either load from a constant
pool or movw + movt into a register.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312521 91177308-0d34-0410-b5e6-96231b3b80d8