loader: Fix ELF relocation logic for Linux v6.17+ startup code layout

[namancse]

After Linux v6.17, the x86_64 kernel startup code moved from .head.text
to .init.text. This moves the startup code to an address, higher than
the actual physical load address. Update the loader to check the
lowest program header address for relocation, not just the entry point,
to handle both old and new kernels.

This issue was identified when latest upstream kernel is built with latest openvmm.
Compilation error:
Error: underhill kernel loader

Caused by:
0: kernel load error
1: elf loader error
2: invalid ELF program header memory offset 134217728, below start 136314880

Tested the change, for bootup, both with current mainline and latest upstream kernel.

[Copilot]

Pull Request Overview

This PR updates the ELF loader logic to handle new Linux kernel layouts introduced in kernel version 6.17. The key change is determining the load offset for Position-Independent Code (PIC) kernels by examining the lowest physical address in program headers instead of relying solely on the entry point address.

• Moved program header parsing earlier in the function to enable load offset calculation
• Changed PIC load offset calculation to use the lowest p_paddr from PT_LOAD segments instead of comparing e_entry with start_address
• Added comprehensive documentation explaining the rationale for both old (< v6.17) and new (≥ v6.17) kernel layouts