Fix MIPS n64 ELF relocation parsing

[plusls]

MIPS n64 (ELFCLASS64 + EM_MIPS) uses a non-standard on-disk r_info layout: r_info = r_sym[4] | r_ssym[1] | r_type3[1] | r_type2[1] | r_type[1] instead of the generic ELF64 (sym << 32) | type. LIEF decoded all ELF64 relocations with the generic mask, which is wrong for MIPS n64:

• Little-endian: r_sym is in the low 32 bits, so r_info >> 32 produces garbage symbol indices. This caused the AUTO dynsym count to explode (196 instead of 10) and relocation types to decode as UNKNOWN.

• Big-endian: r_info & 0xffffffff drags r_ssym/r_type3/r_type2 into the type field. Compound relocations (e.g. R_MIPS_GPREL16 with non-zero Type2/Type3) decode as garbage; simple ones match by accident.

The MIPS n64 encode/decode is now isolated in an internal header src/ELF/reloc_utils.hpp, keeping the public Relocation class format-agnostic (same pattern as the existing Android SLEB128 handling at the Parser/Builder boundary).

The judgment key arch == MIPS && class == ELF64 matches binutils ABI_64_P macro. n32/o32 are ELFCLASS32 and handled by the Elf32 code path at compile time, so they cannot trigger the MIPS n64 branch.

Non-MIPS architectures: zero change in decode/encode paths.