refactor: move x86 ELF loader from src/kernel/elf.c to src/arch/x86/elf.c

refactor: move x86 ELF loader from src/kernel/elf.c to src/arch/x86/elf.c

The ELF loader uses x86-specific page table manipulation (recursive
mapping at 0xFFFFF000/0xFFC00000), EM_386 validation, and low-16MB
allocation. It is 100% architecture-dependent and should not live
in the generic kernel directory.

- Move full implementation to src/arch/x86/elf.c (no #if guards)
- Replace src/kernel/elf.c with a weak stub returning -1
- The linker picks the arch-specific version when available

Passes: make, cppcheck, QEMU smoke test.