[llvm-readobj] Display sections that do not belong to a segment in the section-mapping

Summary:
The following patch adds the "None" line to the section to segment mapping dump.
That line lists the sections that do not belong to any segment.
I realize that this change differs from GNU readelf which does not display the latter information.

I'd rather not add this "feature" under a command line option.  I think that might introduce confusion, since users would have to
make an additional decision as to if they want to see all of the section-to-segment map or just a subset of it.

Another option is to only print the "None" line if the `--section-mapping` option is passed; however,
that might also introduce some confusion, because the section-to-segment map would be different between`--program-headers`
and the `--section-mapping` output.  While the difference is just the "None" line, it seems that if we choose to display
the segment-to-section mapping, then we should always display the whole map including the sections
that do not belong to segments.

```
Section to Segment mapping:
  Segment Sections...
   00
   01     .interp
   02     .interp .note.ABI-tag .gnu.hash
   03     .init_array .fini_array .dynamic
   04     .dynamic
   05     .note.ABI-tag
   06     .eh_frame_hdr
   07
   08     .init_array .fini_array .dynamic .got
   None   .comment .symtab .strtab .shstrtab <--- THIS LINE
```

Reviewers: grimar, rupprecht, jhenderson, espindola

Reviewed By: rupprecht

Subscribers: khemant, emaste, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D57700

llvm-svn: 353217
diff --git a/llvm/test/tools/llvm-readobj/gnu-phdrs.test b/llvm/test/tools/llvm-readobj/gnu-phdrs.test
index ee19650..f6840e5 100644
--- a/llvm/test/tools/llvm-readobj/gnu-phdrs.test
+++ b/llvm/test/tools/llvm-readobj/gnu-phdrs.test
@@ -59,6 +59,7 @@
 ELF32-NEXT:    07     .eh_frame_hdr
 ELF32-NEXT:    08
 ELF32-NEXT:    09     .tdata .ctors .dtors .jcr .dynamic .got
+ELF32-NEXT:    None   .comment .shstrtab .symtab .strtab
 
 ELF64-PHDRS: Elf file type is EXEC (Executable file)
 ELF64-PHDRS-NEXT: Entry point 0x400610
@@ -90,6 +91,7 @@
 ELF64-MAPPING-NEXT:    07     .eh_frame_hdr
 ELF64-MAPPING-NEXT:    08
 ELF64-MAPPING-NEXT:    09     .tdata .init_array .fini_array .jcr .dynamic .got
+ELF64-MAPPING-NEXT:    None   .comment .shstrtab .symtab .strtab
 
 ELF64-ONEMAPPING: Section to Segment mapping:
 ELF64-ONEMAPPING-NOT: Section to Segment mapping: