libdrgn: Always fall back to libdwfl ELF relocation

Simply doing "drgn" segmentation-faults on my machine:

(gdb) bt
 #0  apply_elf_rela_x86_64 (relocating=relocating@entry=0x7fc843650c90, r_offset=0, r_type=2, r_addend=0,
     sym_value=<optimized out>) at ../../libdrgn/arch_x86_64.c:498
 #1  0x00007fc88cbd523b in relocate_elf_section (platform=0x7fc843650c80, shdrnum=59, sh_addrs=0x7fc808000b20,
     symtab_scn=<optimized out>, reloc_scn=0x17614c8, scn=<optimized out>) at ../../libdrgn/debug_info.c:761
 osandov#2  relocate_elf_file (elf=<optimized out>) at ../../libdrgn/debug_info.c:865
 osandov#3  drgn_debug_info_find_sections (module=<optimized out>) at ../../libdrgn/debug_info.c:883
 osandov#4  drgn_debug_info_read_module (load=load@entry=0x7ffea9d70870, dindex_state=0x7ffea9d70810, head=0x163c6b0)
     at ../../libdrgn/debug_info.c:970
 osandov#5  0x00007fc88cbd5474 in drgn_debug_info_update_index._omp_fn.1 () at ../../libdrgn/debug_info.c:1037
 osandov#6  0x00007fc88cb19769 in ?? () from /usr/lib/x86_64-linux-gnu/libgomp.so.1
 osandov#7  0x00007fc88cb21f00 in ?? () from /usr/lib/x86_64-linux-gnu/libgomp.so.1
 osandov#8  0x00007fc88cb1f7aa in ?? () from /usr/lib/x86_64-linux-gnu/libgomp.so.1
 osandov#9  0x00007fc88dfb7fa3 in start_thread (arg=<optimized out>) at pthread_create.c:486
 osandov#10 0x00007fc88dafe4cf in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Always returning NULL in libdrgn/debug_info.c:relocate_elf_file() fixes
the issue.  I don't know, maybe it's a bug in drgn's implementation of
ELF relocation, but add this hack for now.

Signed-off-by: Peilin Ye <[email protected]>

libdrgn/debug_info.c

@@ -1615,6 +1615,7 @@ static struct drgn_error *relocate_elf_section(Elf_Scn *scn, Elf_Scn *reloc_scn,
 static struct drgn_error *relocate_elf_file(Elf *elf)
 {
 	struct drgn_error *err;
+	bool hack = true;
 
 	GElf_Ehdr ehdr_mem, *ehdr;
 	ehdr = gelf_getehdr(elf, &ehdr_mem);
@@ -1628,7 +1629,7 @@ static struct drgn_error *relocate_elf_file(Elf *elf)
 
 	struct drgn_platform platform;
 	drgn_platform_from_elf(ehdr, &platform);
-	if (!platform.arch->apply_elf_rela) {
+	if (!platform.arch->apply_elf_rela || hack) {
 		/* Unsupported; fall back to libdwfl. */
 		return NULL;
 	}