Add symbolizer option to generate debug information only.

There is no need to copy the .rodata and .text sections since the
consumers can get their data form the original ELF file. We mark
those section as SHT_NOBITS instead, which is the standard way to
create ELF file with separate debug information only.

The new option is called --only-keep-debug and it essentially has
the same effect as --only-keep-debug option for "strip" command.

Change-Id: I4f7488c04bbad86288beeda11c3bf295890e740d
diff --git a/compiler/elf_builder.h b/compiler/elf_builder.h
index ef44a6f..26ab281 100644
--- a/compiler/elf_builder.h
+++ b/compiler/elf_builder.h
@@ -841,7 +841,7 @@
       load.p_type    = PT_LOAD;
       load.p_flags   = PF_R;
       load.p_offset  = load.p_vaddr = load.p_paddr = 0;
-      load.p_filesz  = load.p_memsz = sections_[0]->header_.sh_offset;
+      load.p_filesz  = load.p_memsz = sizeof(Elf_Ehdr) + sizeof(Elf_Phdr) * kMaxProgramHeaders;
       load.p_align   = kPageSize;
       phdrs.push_back(load);
     }