[PATCH] D149751: [LLD][ELF][RISCV] Add auipc->lui relaxation.

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This relaxation enables use of weak undefined symbols in
programms written with medany code model in mind if code
section happens to be placed far from zero address(>2GiB).

It performs following transormation:

  auipc + add -> lui + add

It does it if symbol address is out of range for pc-relative
addressing mode (>2GiB far from pc) and within range for
zero-relative one.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D149751

Files:
  lld/ELF/Arch/RISCV.cpp
  lld/test/ELF/riscv-relax-weak.s

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