[RISCV] Support global address as inline asm memory operand of `m`

In D146245, we have supported lowering inline asm `m` with offset
to `register+imm`, but we didn't handle the case that the offset
is the low part of global address.

This patch will emit `%lo(g)` when `g` is a global address.

Fixes #64656

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D157839