Implement `--append-null-phdr` option

[matheusmoreira]

Appends PT_NULL entries to the program header table so that can may be easily targeted by custom tools. Also add automated tests for it. Releated issue: #533.

It works but I can't figure out how to adjust the PT_LOAD segments to cover the expanded PT_PHDR size. Can you give me some pointers?

Output of readelf --file-header --program-headers patched:

ELF Header:
  Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
  Class:                             ELF64
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           AArch64
  Version:                           0x1
  Entry point address:               0x20284c
  Start of program headers:          64 (bytes into file)
  Start of section headers:          201432 (bytes into file)
  Flags:                             0x0
  Size of this header:               64 (bytes)
  Size of program headers:           56 (bytes)
  Number of program headers:         9
  Size of section headers:           64 (bytes)
  Number of section headers:         16
  Section header string table index: 14

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  PHDR           0x0000000000000040 0x0000000000000040 0x0000000000000040
                 0x00000000000001f8 0x00000000000001f8  R      0x8
readelf: Error: the PHDR segment is not covered by a LOAD segment
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RW     0x0
  LOAD           0x0000000000000000 0x0000000000200000 0x0000000000200000
                 0x000000000000184c 0x000000000000184c  R      0x1000
  LOAD           0x000000000000184c 0x000000000020284c 0x000000000020284c
                 0x000000000000b550 0x000000000000b550  R E    0x1000
  LOAD           0x000000000000cda0 0x000000000020eda0 0x000000000020eda0
                 0x00000000000000b8 0x00000000000000b8  RW     0x1000
  GNU_RELRO      0x000000000000cda0 0x000000000020eda0 0x000000000020eda0
                 0x00000000000000b8 0x0000000000000260  R      0x1
  LOAD           0x000000000000ce58 0x000000000020fe58 0x000000000020fe58
                 0x0000000000001358 0x000000000010136d  RW     0x1000
  NULL           0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000         0x0
  NULL           0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000         0x0

 Section to Segment mapping:
  Segment Sections...
   00
   01
   02     .rodata
   03     .text
   04     .data.rel.ro .got
   05     .data.rel.ro .got
   06     .data .bss
   07
   08