[RISCV] Add new lga pseudoinstruction

This mirrors lla and is always GOT-relative, allowing an explicit
request to use the GOT without having to expand the instruction. This
then means la is just defined in terms of lla and lga in the assembler,
based on whether PIC is enabled, and at the codegen level we replace la
entirely with lga since we only ever use la there when we want to load
from the GOT (and assert that to be the case).

See https://github.com/riscv-non-isa/riscv-asm-manual/issues/50

Reviewed By: asb, MaskRay

Differential Revision: https://reviews.llvm.org/D107278