Add PF_W to PT_LOAD segment where the .dynamic section is placed

[Ponsuke07]

About this PR

This PR fix #146.
In some binaries, the .dynamic section may be located in a read-only PT_LOAD segment.
Therefore, add a writable flag to the PT_LOAD segment in which the .dynamic section is located.

How to test

1. Reproduce the Issue

Use the repository from #146 (comment) to build a problematic binary:

# Clone the sample repository
$ git clone https://github.com/limeytexan/NixOS-patchelf-issue-146.git
$ cd NixOS-patchelf-issue-146

# Download dependencies
$ go mod download github.com/sirupsen/logrus
$ go get github.com/sirupsen/logrus@v1.6.0

# Build the binary
$ go build

# Attempt to patchelf
$ patchelf --set-rpath hogehgoe ./issue_146 --output issue_146.patched
$ ./issue_146.patched
Segmentation fault

2. Verify the Fix

After applying this PR, you will see that a writable flag(PF_W) has been added to the PT_LOAD section in which the .dynamic is located.

$: ./issue_146.patched && echo "WORKED" 
WORKED
$ readelf -l ./issue_146.patched 

Elf file type is EXEC (Executable file)
Entry point 0x465100
There are 10 program headers, starting at offset 64

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  PHDR           0x0000000000000040 0x0000000000400040 0x0000000000400040
                 0x0000000000000230 0x0000000000000230  R      0x1000
  TLS            0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000008  R      0x8
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RW     0x8
  LOOS+0x5041580 0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000         0x8
  LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
                 0x0000000000129030 0x0000000000129030  RWE    0x1000 ★PF_W flag has been added
  NOTE           0x0000000000000278 0x0000000000400278 0x0000000000400278
                 0x0000000000000064 0x0000000000000064  R      0x4
  INTERP         0x00000000000002e0 0x00000000004002e0 0x00000000004002e0
                 0x000000000000001c 0x000000000000001c  R      0x1
      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
  DYNAMIC        0x0000000000000508 0x0000000000400508 0x0000000000400508
                 0x0000000000000130 0x0000000000000130  RW     0x8
  LOAD           0x000000000012a000 0x000000000052a000 0x000000000052a000
                 0x00000000001307d0 0x00000000001307d0  R      0x1000
  LOAD           0x000000000025b000 0x000000000065b000 0x000000000065b000
                 0x0000000000035f60 0x000000000006bf08  RW     0x1000

 Section to Segment mapping:
  Segment Sections...
   00     
   01     
   02     
   03     
   04     .note.go.buildid .interp .dynstr .dynamic .text .plt 
   05     .note.go.buildid 
   06     .interp 
   07     .dynamic 
   08     .rodata .rela .rela.plt .gnu.version .gnu.version_r .hash .dynsym .typelink .itablink .gosymtab .gopclntab 
   09     .go.buildinfo .got.plt .got .noptrdata .data .bss .noptrbss

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It also confirms that make check tests pass.