Fix incorrect .debug_frame lookup with non-zero text segment vaddr

Unwinding over ptrace and unwinding coredump fail to lookup the
.debug_frame dwarf data when the ELF file text segment virtual address
is non-zero. Looking at some binaries, the virtual address is non-zero
for non-pie binaries, and zero for PIC shared libraries and PIE
executables.

The core dump unwinder can be used for demonstrating the bug. Without
this patch, the unwinding fails badly (testing with a ARM qemu image):

 $ UNW_ARM_UNWIND_METHOD=1 ./test-coredump-unwind core `cat backing_files`
 test-coredump-unwind: unw_get_proc_info(ip=0x86d8) failed: ret=-10

After applying this patch, we can unwind all the way until running out
of dwarf data:

 $ UNW_ARM_UNWIND_METHOD=1 ./test-coredump-unwind core `cat backing_files`
     ip=0x000086d8 proc=000086d4-000086dc handler=0x00000000 lsda=0x00000000
 test-coredump-unwind: step
 test-coredump-unwind: step done:1
     ip=0x000086ef proc=000086dc-000086f2 handler=0x00000000 lsda=0x00000000
 test-coredump-unwind: step
 test-coredump-unwind: step done:1
     ip=0x000086e7 proc=000086dc-000086f2 handler=0x00000000 lsda=0x00000000
 test-coredump-unwind: step
 test-coredump-unwind: step done:1
     ip=0x00008597 proc=00008584-0000859a handler=0x00000000 lsda=0x00000000
 test-coredump-unwind: step
 test-coredump-unwind: step done:1
     ip=0x76eacc3b proc=76eacba0-76eaccec handler=0x00000000 lsda=0x00000000
 test-coredump-unwind: step
 test-coredump-unwind: step done:1
 test-coredump-unwind: unw_get_proc_info(ip=0x85c3) failed: ret=-10

Note how the binary itself is mapped to address 0x8000, the virtual
address for the text segment is 0x8000, and the .debug_frame program
counter values are relative to 0:

$ tr ' ' '\n' < backing_files
0x8000:/home/user/tests/crasher
0x76e96000:/lib/arm-linux-gnueabi/libc-2.13.so
0x76f77000:/lib/arm-linux-gnueabi/libgcc_s.so.1
0x76f88000:/lib/arm-linux-gnueabi/ld-2.13.so

$ readelf -l crasher
Elf file type is EXEC (Executable file)
Entry point 0x859d
There are 9 program headers, starting at offset 52

Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  EXIDX          0x0007b0 0x000087b0 0x000087b0 0x00030 0x00030 R   0x4
  PHDR           0x000034 0x00008034 0x00008034 0x00120 0x00120 R E 0x4
  INTERP         0x000154 0x00008154 0x00008154 0x00013 0x00013 R   0x1
      [Requesting program interpreter: /lib/ld-linux.so.3]
  LOAD           0x000000 0x00008000 0x00008000 0x007e4 0x007e4 R E 0x8000
  LOAD           0x000efc 0x00010efc 0x00010efc 0x00148 0x00154 RW 0x8000
  DYNAMIC        0x000f08 0x00010f08 0x00010f08 0x000f8 0x000f8 RW  0x4
  NOTE           0x000168 0x00008168 0x00008168 0x00044 0x00044 R   0x4
  GNU_STACK      0x000000 0x00000000 0x00000000 0x00000 0x00000 RW  0x4
  GNU_RELRO      0x000efc 0x00010efc 0x00010efc 0x00104 0x00104 R   0x1

$ readelf --debug-dump=frames crasher | grep FDE
00000010 00000024 00000000 FDE cie=00000000 pc=00008614..000086d4
00000038 0000000c 00000000 FDE cie=00000000 pc=000086d4..000086dc
00000048 00000014 00000000 FDE cie=00000000 pc=000086dc..000086f2
00000060 00000014 00000000 FDE cie=00000000 pc=00008584..0000859a

src/dwarf/Gfind_unwind_table.c

@@ -220,7 +220,7 @@ dwarf_find_unwind_table (struct elf_dyn_info *edi, unw_addr_space_t as,
 
 #ifdef CONFIG_DEBUG_FRAME
   /* Try .debug_frame. */
-  found = dwarf_find_debug_frame (found, &edi->di_debug, ip, segbase, path,
+  found = dwarf_find_debug_frame (found, &edi->di_debug, ip, load_base, path,
 				  start_ip, end_ip);
 #endif