[RISCV] Implement pseudo instructions for load/store from a symbol address.

Summary:
Those pseudo-instructions are making load/store instructions able to
load/store from/to a symbol, and its always using PC-relative addressing
to generating a symbol address.

Reviewers: asb, apazos, rogfer01, jrtc27

Differential Revision: https://reviews.llvm.org/D50496

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354430 91177308-0d34-0410-b5e6-96231b3b80d8
13 files changed