fix: correct pt_regs offset for kernel unwind

[kylewanginchina]

One or more of:

• Agent

Fixes incorrect pt_regs offset in kernel unwind on Linux 5.15+ (c4cf55e)

Steps to reproduce the bug

• Run the continuous profiler on a Linux 5.15+ kernel with DWARF/unwind enabled
• Observe invalid or incorrect user-mode registers/stacks during kernel unwind (e.g., invalid FP or
  broken stacks)

Changes to fix the bug

• Add a 5.15+ perf-profiler BPF build variant and select it at runtime based on kernel version
• Define helper IDs and wrappers for bpf_get_current_task_btf and bpf_task_pt_regs
• Use bpf_task_pt_regs (with BTF-typed task pointer) to read user pt_regs on 5.15+ and keep the old
  path as fallback

Affected branches

• main

Checklist

• Added unit test to verify the fix.
• Verified eBPF program runs successfully on linux 4.14.x.
• Verified eBPF program runs successfully on linux 4.19.x.
• Verified eBPF program runs successfully on linux 5.2.x.
• Verified eBPF program runs successfully on linux 5.4.x.
• Verified eBPF program runs successfully on linux 5.15.x.