Refine ELF segments

This commit refines the program headers to generate a better ELF
executable file, with the following changes:

- Always create two load segments, regardless of static or dynamic
  linking.
  - The first load segment is readable and executable, and includes
    the read-only sections such as .text, .rodata, etc.
  - The second load segment is readable and writable, and contains the
    remaining sections, including .data and .bss.
  - In dynamic linking mode, the .rel.plt and .plt sections reside in
    the first segment, while the other dynamic sections are placed in
    the second segment.
- Set the alignment value of both load segments to 0x1000 (the default
  page size) to meet the alignment requirement when the ELF interpreter
  loads the executable.
- Adjust the offset and virtual address of the first load segment to
  ensure that dynamically linked executable can be correctly loaded by
  the kernel program loader.
  - Now, the ELF header and program headers are also loaded into the
    first segment but they are never used.
- Update other segments as necessary to reflect the above changes.

Author: DrXiao