Adjust relocations after relaxation on LoongArch

[CSharperMantle]

This PR adds relocation adjustment to discard stale ones after relaxation. For simplicity, this PR does not delete old relocation entries. Instead, we set them to R_LARCH_NONE to make them a no-op.

Fixes #1584. With this applied, relaxed instructions in https://github.com/CSharperMantle/mold-issue-1584 are attached with correct relocations.

$ make CC=clang CFLAGS='--target=loongarch64-unknown-linux-gnu' MOLD=~/workspace/mold-dev/build/mold disasm
clang  --target=loongarch64-unknown-linux-gnu -c repro.s -o repro.o
clang -nostdlib -static --ld-path=/home/csmantle/workspace/mold-dev/build/mold -Wl,--emit-relocs -o repro repro.o
objdump --disassemble --reloc repro

repro:     file format elf64-loongarch


Disassembly of section .text:

0000000000210278 <_start>:
  210278:       19f7ffcc        pcaddi          $t0, -16386
                        210278: R_LARCH_NONE    sym
                        210278: R_LARCH_RELAX   *ABS*
                        210278: R_LARCH_PCREL20_S2      sym
                        210278: R_LARCH_RELAX   *ABS*
  21027c:       19f7ffad        pcaddi          $t1, -16387
                        21027c: R_LARCH_NONE    sym
                        21027c: R_LARCH_RELAX   *ABS*
                        21027c: R_LARCH_PCREL20_S2      sym
                        21027c: R_LARCH_RELAX   *ABS*
  210280:       0381740b        li.w            $a7, 0x5d
  210284:       03800004        li.w            $a0, 0x0
  210288:       002b0000        syscall         0x0

---

v2 (44859df):

• Fix test bugs caused by [lld elf] Segfault within ld-linux in -nostdlib executable llvm/llvm-project#107749.
• Rebase.

[CSharperMantle]

Will look into CI failures today.

[CSharperMantle]

I think we're hitting a case of llvm/llvm-project#107749 here, where an INTERP section appears in the absence of DYNAMICs.

$ loongarch64-linux-gnu-readelf -l build/out/test/loongarch64/arch-loongarch64-emit-relocs-relax/exe

Elf file type is EXEC (Executable file)
Entry point 0x2102d8
There are 10 program headers, starting at offset 64

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  PHDR           0x0000000000000040 0x0000000000200040 0x0000000000200040
                 0x0000000000000230 0x0000000000000230  R      0x8
  INTERP         0x0000000000000270 0x0000000000200270 0x0000000000200270
                 0x0000000000000025 0x0000000000000025  R      0x1
      [Requesting program interpreter: /lib64/ld-linux-loongarch-lp64d.so.1]
  NOTE           0x0000000000000298 0x0000000000200298 0x0000000000200298
                 0x0000000000000024 0x0000000000000024  R      0x4
  LOAD           0x0000000000000000 0x0000000000200000 0x0000000000200000
                 0x00000000000002d8 0x00000000000002d8  R      0x10000
  LOAD           0x00000000000002d8 0x00000000002102d8 0x00000000002102d8
                 0x0000000000000040 0x0000000000000040  R E    0x10000
  LOAD           0x0000000000000318 0x0000000000220318 0x0000000000220318
                 0x0000000000000010 0x000000000000fce8  RW     0x10000
  LOAD           0x0000000000000328 0x0000000000230328 0x0000000000230328
                 0x0000000000000020 0x0000000000000020  RW     0x10000
  GNU_EH_FRAME   0x00000000000002c4 0x00000000002002c4 0x00000000002002c4
                 0x000000000000000c 0x000000000000000c  R      0x4
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RW     0x1
  GNU_RELRO      0x0000000000000318 0x0000000000220318 0x0000000000220318
                 0x0000000000000010 0x000000000000fce8  R      0x1

 Section to Segment mapping:
  Segment Sections...
   00
   01     .interp
   02     .note.gnu.build-id
   03     .interp .note.gnu.build-id .eh_frame .eh_frame_hdr .rodata
   04     .text
   05     .got .relro_padding
   06     .data .got.plt
   07     .eh_frame_hdr
   08
   09     .got .relro_padding

with the following backtrace:

#0  0x00007ffff7fa9ef0 in audit_list_add_dynamic_tag (list=0x7ffffffcecc0, main_map=0x7ffff7fcd270, tag=1879047932) at rtld.c:224                           
#1  dl_main (phdr=<optimized out>, phnum=<optimized out>, user_entry=<optimized out>, auxv=<optimized out>) at rtld.c:1766                                             
#2  0x00007ffff7fa6c54 in _dl_sysdep_start (start_argptr=start_argptr@entry=0x7ffffffcf4c0, dl_main=dl_main@entry=0x7ffff7fa88d0 <dl_main>) at ../sysdeps/unix/sysv/linux/dl-sysdep.c:140                                    
#3  0x00007ffff7fa7964 in _dl_start_final (arg=arg@entry=0x7ffffffcf4c0, info=info@entry=0x7ffffffcefd0) at rtld.c:496
#4  0x00007ffff7fa85cc in _dl_start (arg=0x7ffffffcf4c0) at rtld.c:583
#5  0x00007ffff7fa724c in _start () from /lib64/ld-linux-loongarch-lp64d.so.1

I'll add --static as a workaround in v2, as mentioned in llvm/llvm-project#107749.