[obj2yaml] Emit ProgramHeader.Offset

Currently, obj2yaml doesn't emit the offset of program headers, leaving
it to yaml2obj to calculate offsets based on `FirstSec` and `LastSec`.
This causes an obj2yaml->yaml2obj round trip to often produce an ELF
file that is not equivalent to the original, especially since it seems
common to have program headers at offset 0 whose first section starts at
a higher address. Besides being non-equivalent, the produced ELF files
also do not seem to work propery and readelf complains about them.

Taking a simple hello world program in C, compiled using either GCC or
Clang, the original ELF file has the following program headers (only
showing some relevant ones):

```
Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  PHDR           0x0000000000000040 0x0000000000000040 0x0000000000000040
                 0x00000000000002d8 0x00000000000002d8  R      0x8
  INTERP         0x0000000000000318 0x0000000000000318 0x0000000000000318
                 0x000000000000001c 0x000000000000001c  R      0x1
      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
  LOAD           0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000630 0x0000000000000630  R      0x1000
  LOAD           0x0000000000001000 0x0000000000001000 0x0000000000001000
                 0x0000000000000161 0x0000000000000161  R E    0x1000
...

 Section to Segment mapping:
  Segment Sections...
   00
   01     .interp
   02     .interp .note.gnu.property .note.gnu.build-id .note.ABI-tag .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt
   03     .init .plt .text .fini
...
```

While this is the result of an obj2yaml->yaml2obj round trip:

```
Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  PHDR           0x0000000000000000 0x0000000000000040 0x0000000000000040
                 0x0000000000000000 0x0000000000000000  R      0x8
readelf: Error: the PHDR segment is not covered by a LOAD segment
  INTERP         0x0000000000000318 0x0000000000000318 0x0000000000000318
                 0x000000000000001c 0x000000000000001c  R      0x1
      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
  LOAD           0x0000000000000318 0x0000000000000000 0x0000000000000000
                 0x0000000000000318 0x0000000000000318  R      0x1000
  LOAD           0x0000000000001000 0x0000000000001000 0x0000000000001000
                 0x0000000000000161 0x0000000000000161  R E    0x1000
...

 Section to Segment mapping:
  Segment Sections...
   00
   01     .interp
   02
   03     .init .plt .text .fini
...
```

Note that the offset of segment 2 changed from 0x0 to 0x318. This has
two effects:
- readelf complains "Error: the PHDR segment is not covered by a LOAD
  segment" since PHDR was originally covered by segment 2 but not
  anymore;
- Segment 2 effectively became empty according to the section to segment
  mapping.

I addition to these, the output doesn't correctly execute anymore,
crashing with a "SIGSEGV (Address boundary error)".

This patch fixes the difference in program header layout after a round
trip by explicitly emitting offsets.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D145555