Auto merge of #62281 - Disasm:riscv-pic, r=nagisa
Add support for pc-relative addressing on 64-bit RISC-V
These changes allow Rust to generate position-independent code on `riscv64` targets with code model `medium`.
Closes: https://github.com/rust-lang/rust/issues/59802
See also: https://github.com/rust-embedded/riscv-rt/issues/25, https://github.com/rust-embedded/wg/issues/218